Document our issue locking policy (#29433)

Author: delvh

CONTRIBUTING.md

@@ -8,6 +8,7 @@
     - [How to report issues](#how-to-report-issues)
     - [Types of issues](#types-of-issues)
     - [Discuss your design before the implementation](#discuss-your-design-before-the-implementation)
+    - [Issue locking](#issue-locking)
   - [Building Gitea](#building-gitea)
   - [Dependencies](#dependencies)
     - [Backend](#backend)
@@ -103,6 +104,13 @@ the goals for the project and tools.
 
 Pull requests should not be the place for architecture discussions.
 
+### Issue locking
+
+Commenting on closed or merged issues/PRs is strongly discouraged.
+Such comments will likely be overlooked as some maintainers may not view notifications on closed issues, thinking that the item is resolved.
+As such, commenting on closed/merged issues/PRs may be disabled prior to the scheduled auto-locking if a discussion starts or if unrelated comments are posted.
+If further discussion is needed, we encourage you to open a new issue instead and we recommend linking to the issue/PR in question for context.
+
 ## Building Gitea
 
 See the [development setup instructions](https://docs.gitea.com/development/hacking-on-gitea).