MAINTAINING.md: add some notes on Bors

Author: benpicco

MAINTAINING.md

@@ -99,6 +99,30 @@ complete that no input from the original developer or maintainer is required.
 3.  Are critical/hard to understand parts in the code documented?
 4.  Check grammar and spelling of documentation
 
+### Bors usage
+
+RIOT uses [Bors] to merge Pull Requests. Bors can batch up to 4 PRs into a
+merge train and build and merge them all at once.
+This can greatly reduce CI times, but Bors has many quirks:
+
+ - To merge a PR, comment "bors merge" under it.
+   The PR needs to be approved and CI must have already have run (CI: Ready for build)
+   If you write "bors merge" before CI has finished running for the PR, Bors will
+   tell you it will do the merge once CI is finished.
+   This is a lie. You will have to manually write "bors merge" again once everything
+   but Bors is green.
+
+ - If there are multiple PRs that area ready to merge (all green except for Bors) you
+   can combine them into a merge train.
+   Be careful though: If CI is not busy, Bors will already start to build the first PR
+   once you type "Bors merge" and only include the other PRs in the next merge train.
+   To prevent this, you can first schedule a [dummy PR].
+   While the dummy PR (a no-op PR with "CI: skip compile tests" set) is running, you
+   can write "bors merge" under all the PRs you want to merge.
+   Be quick: You have a bit less than 1 minute of time to do this.
+
+ - If Bors gets stuck you can write "bors cancel" under the PRs it last tried to build.
+   This can sometimes happen if you cancel a build in the Web UI.
 
 ## Non-technical guidelines
 
@@ -187,3 +211,5 @@ there.
 [Comparing build sizes]: https://github.com/RIOT-OS/RIOT/wiki/Comparing-build-sizes
 [Coding Conventions]: CODING_CONVENTIONS.md
 [Code of Conduct]: https://github.com/RIOT-OS/RIOT/blob/master/CODE_OF_CONDUCT.md
+[Bors]: https://bors.tech/
+[dummy PR]: https://github.com/RIOT-OS/RIOT/pull/19253