Breaking changes develop merge to master, 2021Q3 edition. (#14196)

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+# ChibiOS Upgrade Procedure
+
+ChibiOS and ChibiOS-Contrib need to be updated in tandem -- the latter has a branch tied to the ChibiOS version in use and should not be mixed with different versions.
+
+## Getting ChibiOS
+
+* `svn` Initialisation:
+    * Only needed to be done once
+    * You might need to separately install `git-svn` package in your OS's package manager
+    * `git svn init --stdlayout --prefix='svn/' http://svn.osdn.net/svnroot/chibios/`
+    * `git remote add qmk git@github.com:qmk/ChibiOS.git`
+* Updating:
+    * `git svn fetch`
+    * First time around this will take several hours
+    * Subsequent updates will be incremental only
+* Tagging example (work out which version first!):
+    * `git tag -a ver20.3.3 -m ver20.3.3 svn/tags/ver20.3.3`
+    * `git push qmk ver20.3.3`
+    * `git tag -a breaking_YYYY_qN -m breaking_YYYY_qN svn/tags/ver20.3.3`
+    * `git push qmk breaking_YYYY_qN`
+
+## Getting ChibiOS-Contrib
+
+* `git` Initialisation:
+    * `git clone git@github.com:qmk/ChibiOS-Contrib`
+    * `git remote add upstream https://github.com/ChibiOS/ChibiOS-Contrib`
+    * `git checkout -b chibios-20.3.x upstream/chibios-20.3.x`
+* Updating:
+    * `git fetch --all --tags --prune`
+    * `git checkout chibios-20.3.x`
+    * `git pull --ff-only`
+    * `git push origin chibios-20.3.x`
+    * `git tag -a breaking_YYYY_qN -m breaking_YYYY_qN chibios-20.3.x`
+    * `git push origin breaking_YYYY_qN`
+
+## Updating submodules
+
+* Update the submodules
+    * `cd $QMK_FIRMWARE`
+    * `git checkout develop`
+    * `git pull --ff-only`
+    * `git checkout -b chibios-version-bump`
+    * `cd lib/chibios`
+    * `git fetch --all --tags --prune`
+    * `git checkout breaking_YYYY_qN`
+    * `cd ../chibios-contrib`
+    * `git fetch --all --tags --prune`
+    * `git checkout breaking_YYYY_qN`
+* Build everything
+    * `cd $QMK_FIRMWARE`
+    * `qmk multibuild -j4`
+    * Make sure there are no errors
+* Push to the repo
+    * `git commit -am 'Update ChibiOS to XXXXXXXXX'`
+    * `git push --set-upstream origin chibios-version-bump`
+* Make a PR to qmk_firmware with the new branch
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