Merge pull request RIOT-OS#12500 from maribu/maintainer-doc

MAINTAINING.md: Added doc on release process

MAINTAINING.md

@@ -155,6 +155,28 @@ GitHub's review sidebar.
 If there are multiple maintainers reviewing a PR, always give the other
 maintainers reasonable time to ACK before dismissing their review.
 
+### Release, Feature Freeze, and Backports
+
+Before the official release of a new RIOT version, two feature freeze periods
+are announced on the
+[RIOT development email list](https://lists.riot-os.org/mailman/listinfo/devel):
+The soft feature freeze and the hard feature freeze. During the soft feature
+freeze only PRs with minor impact should be merged into master. The hard feature
+freeze begins when the release manager creates a new release branch. Therefore,
+the restriction on merging PRs into the master branch are lifted at that point.
+
+Once the release branch is created, no backports of new features will be
+accepted. Instead, backports should consist only of bug fixes or of reverting
+features that were added during the last development period (and not part of any
+release), but didn't reach the required maturity or API stability yet. For
+bigger changes (which explicitly includes any revert), the PR has to be
+announced to the maintainer mailing list and should be merged no sooner than
+48h after the announcement and needs at least two ACKs.
+
+In case of security relevant backports (both bug fixes and reverts), the
+announcement can be skipped and the fix merged once at least two ACKs are
+there.
+
 [list of maintainers]: https:/RIOT-OS/RIOT/wiki/Maintainers
 [Best Practices]: https:/RIOT-OS/RIOT/wiki/Best-Practice-for-RIOT-Programming
 [Comparing build sizes]: https:/RIOT-OS/RIOT/wiki/Comparing-build-sizes