Document that the retro and kickoff notes are public

CONTRIBUTING.md

@@ -88,6 +88,27 @@ contributing to GitLab.
 
 Please see the [UX Guide for GitLab].
 
+## Release retrospective and kickoff
+
+### Retrospective
+
+After each release (usually on the 22nd of each month), we have a retrospective
+call where we discuss what went well, what went wrong, and what we can improve
+for the next release. The [retrospective notes] are public and you are invited
+to comment them.
+If you're interested, you can even join the [retrospective call][retro-kickoff-call].
+
+### Kickoff
+
+Before working on the next release (usually on the 8th of each month), we have a
+kickoff call to explain what we expect to ship in the next release. The
+[kickoff notes] are public and you are invited to comment them.
+If you're interested, you can even join the [kickoff call][retro-kickoff-call].
+
+[retrospective notes]: https://docs.google.com/document/d/1nEkM_7Dj4bT21GJy0Ut3By76FZqCfLBmFQNVThmW2TY/edit?usp=sharing
+[kickoff notes]: https://docs.google.com/document/d/1ElPkZ90A8ey_iOkTvUs_ByMlwKK6NAB2VOK5835wYK0/edit?usp=sharing
+[retro-kickoff-call]: https://gitlab.zoom.us/j/918821206
+
 ## Issue tracker
 
 To get support for your particular problem please use the