docs: add RELEASE.md and CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to the chatmail relay
+
+Commit messages follow the [Conventional Commits] notation.
+We use [git-cliff] to generate the changelog from commit messages before the release.
+
+[Conventional Commits]: https://www.conventionalcommits.org/
+[git-cliff]: https://git-cliff.org/

RELEASE.md

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+# Releasing a new version of chatmail relay
+
+For example, to release version 1.9.0 of chatmail relay, do the following steps.
+
+1. Update the changelog: `git cliff --unreleased --tag 1.9.0 --prepend CHANGELOG.md` or `git cliff -u -t 1.9.0 -p CHANGELOG.md`.
+
+2. Open the changelog in the editor, edit it if required.
+
+3. Commit the changes to the changelog with a commit message `chore(release): prepare for 1.9.0`.
+
+3. Tag the release: `git tag --annotate 1.9.0`.
+
+4. Push the release tag: `git push origin 1.9.0`.
+
+5. Create a GitHub release: `gh release create 1.9.0`.