Add CONTRIBUTE.md

CONTRIBUTE.md

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+# Notes to contribute to the charts
+
+## Version bump
+
+Each merge request should include a version bump.
+
+Example:
+
+The MR brings a small fix. `main` is at `0.2.0`.  
+Then it should include a change that bumps the version number in `Chart.yaml` to
+`0.2.1`.
+
+## Changelog
+
+Each merge request should include a changelog.  
+For examples, have a look at the existing `CHANGELOG.md` files.
+
+The suggested format is the following.
+
+```
+## x.y.z
+
+**Changes**
+
+List notable changes here...
+
+**Migration notes**
+
+List migration steps here...
+Omit if unnecessary.
+
+**Commits**
+
+$(git shortlog main..)
+```