Add checklist for backporting a PR

Signed-off-by: Richard Steinmetz <richard@steinmetz.cloud>

processes/backport_checklist.md

@@ -0,0 +1,44 @@
+# Checklist for Backporting Pull Requests
+
+1. Determine which Nextcloud versions are supported by visiting the [maintenance schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule).
+2. Determine the branch of the app corresponding to the supported Nextcloud versions.
+    1. Go to the app store page of the app, e.g. [Mail](https://apps.nextcloud.com/apps/mail).
+    2. Scroll down to `Downloads`.
+    3. Have a look at the version table and take note of our app version.
+    4. Derive the branch name by using the following pattern: `stable{MAJOR}.{MINOR}` (replace
+    `{MAJOR}` and `{MINOR}` with their respective versions)
+3. Instruct the backport bot by commenting on the pull request with `/backport to stableX.X`.
+4. Done! Now, wait for the bot to do its work. It will automatically open a new pull request with
+the backport.
+5. *Optional:* In some cases the backport bot will fail and won't crate a PR.
+Instead, it will leave a comment stating the error.
+You have to cherry pick the commits and create a new PR manually in those cases.
+
+### Example
+
+Let's assume that Nextcloud 23 - 25 are supported according to the maintenance schedule and the app
+store contains the following table:
+
+|              |       |
+| ------------ | ----- |
+| Nextcloud 25 | 2.0.3 |
+| Nextcloud 24 | 1.14.1|
+| Nextcloud 23 | 1.14.1|
+
+That means that we need to backport to `stable2.0` and `stable1.14`.
+
+## Exceptions
+
+These rules apply in most cases.
+However, there are some exceptions to the rules.
+
+### Exception: Recent features
+
+If a feature is fairly new it might not have to be backported to all supported stable branches.
+For example, a fix for a feature added in v2.1 has to be backported to `stable2.1` and all branches
+after that (e.g. `stable3.0`).
+
+When in doubt, you can search for the PR implementing a feature and consult its milestone.
+The milestone is named after the version a feature was introduced.
+Bug reports also often contain the version of an app on which the bug occurred.
+Just check the issue you fixed and look for the app version.