More PR checklist updates (#14705)

* Wording, clarification.

* Apply suggestions from code review

Co-authored-by: Mikkel Jeppesen <2756925+Duckle29@users.noreply.github.com>

Co-authored-by: Mikkel Jeppesen <2756925+Duckle29@users.noreply.github.com>

docs/pr_checklist.md

@@ -4,14 +4,14 @@ This is a non-exhaustive checklist of what the QMK Collaborators will be checkin
 
 If there are any inconsistencies with these recommendations, you're best off [creating an issue](https://github.com/qmk/qmk_firmware/issues/new) against this document, or getting in touch with a QMK Collaborator on [Discord](https://discord.gg/Uq7gcHh).
 
-## General PRs
+## Requirements for all PRs
 
 - PR should be submitted using a non-`master` branch on the source repository
     - this does not mean you target a different branch for your PR, rather that you're not working out of your own master branch
     - if submitter _does_ use their own `master` branch, they'll be given a link to the ["how to git"](https://docs.qmk.fm/#/newbs_git_using_your_master_branch) page after merging -- (end of this document will contain the contents of the message)
 - newly-added directories and filenames must be lowercase
-    - this rule may be relaxed if upstream sources originally had uppercase characters (e.g. ChibiOS, or imported files from other repositories etc.)
+    - this rule may be relaxed if upstream sources originally had uppercase characters (e.g. LUFA, ChibiOS, or imported files from other repositories etc.)
-    - if there is enough justification (i.e. consistency with existing core files etc.) this can be relaxed
+    - if there is valid justification (i.e. consistency with existing core files etc.) this can be relaxed
         - a board designer naming their keyboard with uppercase letters is not enough justification
 - valid license headers on all `*.c` and `*.h` source files
     - GPL2/GPL3 recommended for consistency
@@ -21,7 +21,7 @@ If there are any inconsistencies with these recommendations, you're best off [cr
 - QMK Codebase "best practices" followed
     - this is not an exhaustive list, and will likely get amended as time goes by
     - `#pragma once` instead of `#ifndef` include guards in header files
-    - no "old-school" GPIO/I2C/SPI functions used -- must use QMK abstractions unless justifiable (and laziness is not valid justification)
+    - no "old-school" or other low-level GPIO/I2C/SPI functions may be used -- must use QMK abstractions unless justifiable (and laziness is not valid justification)
     - timing abstractions should be followed too:
         - `wait_ms()` instead of `_delay_ms()` (remove `#include <util/delay.h>` too)
         - `timer_read()` and `timer_read32()` etc. -- see [timer.h](https://github.com/qmk/qmk_firmware/blob/master/tmk_core/common/timer.h) for the timing APIs
@@ -30,7 +30,7 @@ If there are any inconsistencies with these recommendations, you're best off [cr
         - discuss it with QMK Collaborators on Discord
         - refactor it as a separate core change
         - remove your specific copy in your board
-- rebase and fix all merge conflicts before opening the PR (in case you need help or advice, reach out to QMK Collaborators on Discord)
+- fix all merge conflicts before opening the PR (in case you need help or advice, reach out to QMK Collaborators on Discord)
 
 ## Keymap PRs
 
@@ -50,11 +50,13 @@ https://github.com/qmk/qmk_firmware/pulls?q=is%3Apr+is%3Aclosed+label%3Akeyboard
     - valid maintainer
     - displays correctly in Configurator (press Ctrl+Shift+I to preview local file, turn on fast input to verify ordering)
 - `readme.md`
-    - standard template should be present
+    - standard template should be present -- [link to template](https://github.com/qmk/qmk_firmware/blob/master/data/templates/avr/readme.md)
-    - flash command has `:flash` at end
+    - flash command is present, and has `:flash` at end
     - valid hardware availability link (unless handwired) -- private groupbuys are okay, but one-off prototypes will be questioned. If open-source, a link to files should be provided.
     - clear instructions on how to reset the board into bootloader mode
     - a picture about the keyboard and preferably about the PCB, too
+        - images are not to be placed in the `qmk_firmware` repository
+        - images should be uploaded to an external image hosting service, such as [imgur](https://imgur.com/).
 - `rules.mk`
     - removed `MIDI_ENABLE`, `FAUXCLICKY_ENABLE` and `HD44780_ENABLE`
     - modified `# Enable Bluetooth with the Adafruit EZ-Key HID` -> `# Enable Bluetooth`
@@ -71,20 +73,20 @@ https://github.com/qmk/qmk_firmware/pulls?q=is%3Apr+is%3Aclosed+label%3Akeyboard
         - initialisation code for the matrix and critical devices
         - mirroring existing functionality of a commercial board (like custom keycodes and special animations etc.) should be handled through non-`default` keymaps
     - Vial-related files or changes will not be accepted, as they are not used by QMK firmware (no Vial-specific core code has been submitted or merged)
-- `keyboard.c`
+- `<keyboard>.c`
     - empty `xxxx_xxxx_kb()` or other weak-defined default implemented functions removed
     - commented-out functions removed too
     - `matrix_init_board()` etc. migrated to `keyboard_pre_init_kb()`, see: [keyboard_pre_init*](https://docs.qmk.fm/#/custom_quantum_functions?id=keyboard_pre_init_-function-documentation)
     - prefer `CUSTOM_MATRIX = lite` if custom matrix used, allows for standard debounce, see [custom matrix 'lite'](https://docs.qmk.fm/#/custom_matrix?id=lite)
     - prefer LED indicator [Configuration Options](https://docs.qmk.fm/#/feature_led_indicators?id=configuration-options) to custom `led_update_*()` implementations where possible
-- `keyboard.h`
+- `<keyboard>.h`
     - `#include "quantum.h"` appears at the top
     - `LAYOUT` macros should use standard definitions if applicable
         - use the Community Layout macro names where they apply (preferred above `LAYOUT`/`LAYOUT_all`)
 - keymap `config.h`
     - no duplication of `rules.mk` or `config.h` from keyboard
 - `keymaps/default/keymap.c`
-    - `QMKBEST`/`QMKURL` removed (sheesh)
+    - `QMKBEST`/`QMKURL` removed
     - if using `MO(_LOWER)` and `MO(_RAISE)` keycodes or equivalent, and the keymap has an adjust layer when holding both keys -- if the keymap has no "direct-to-adjust" keycode (such as `MO(_ADJUST)`) then you should prefer to write...
         ```
         layer_state_t layer_state_set_user(layer_state_t state) {
@@ -100,7 +102,6 @@ https://github.com/qmk/qmk_firmware/pulls?q=is%3Apr+is%3Aclosed+label%3Akeyboard
 - submitters can also have a "manufacturer-matching" keymap that mirrors existing functionality of the commercial product, if porting an existing board
 - Do not include VIA json files in the PR. These do not belong in the QMK repository as they are not used by QMK firmware -- they belong in the [VIA Keyboard Repo](https://github.com/the-via/keyboards)
 
-
 Also, specific to ChibiOS:
 - **strong** preference to using existing ChibiOS board definitions.
     - a lot of the time, an equivalent Nucleo board can be used with a different flash size or slightly different model in the same family
@@ -114,7 +115,7 @@ Also, specific to ChibiOS:
 ## Core PRs
 
 - must now target `develop` branch, which will subsequently be merged back to `master` on the breaking changes timeline
-- other notes TBD
+- other requirements are at the discretion of QMK collaborators
     - core is a lot more subjective given the breadth of posted changes
 
 ---
@@ -159,3 +160,10 @@ Additionally, PR reviews are something that is done in our free time. We are not
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 ```
+
+Or, optionally, using [SPDX identifier](https://spdx.org/licenses/) instead:
+
+```
+// Copyright 2021 Your Name (@yourgithub)
+// SPDX-License-Identifier: GPL-2.0-or-later
+```