docs: document release process in RELEASE.md

Author: link2xt

RELEASE.md

@@ -0,0 +1,18 @@
+# Releasing a new version of DeltaChat core
+
+For example, to release version 1.116.0 of the core, do the following steps.
+
+1. Resolve all [blocker issues](https://github.com/deltachat/deltachat-core-rust/labels/blocker).
+
+2. Update the changelog: `git cliff --unreleased --tag 1.116.0 --prepend CHANGELOG.md` or `git cliff -u -t 1.116.0 -p CHANGELOG.md`.
+
+3. Update the version by running `scripts/set_core_version.py 1.116.0`.
+
+4. Commit the changes as `chore(release): prepare for 1.116.0`.
+   Optionally, use a separate branch like `prep-1.116.0` for this commit and open a PR for review.
+
+5. Tag the release: `git tag -a v1.116.0`.
+
+6. Push the release tag: `git push origin v1.116.0`.
+
+7. Create a GitHub release: `gh release create v1.116.0 -n ''`.