Add release docs

RELEASE.md

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+# Releasing `circles-payment-api`
+
+Use this checklist to create a new release of `circles-payment-api`. All steps are intended to be run from the root directory of the repository.
+
+## Creating a new release
+
+1. Make sure you are currently on the `main` branch, otherwise run `git checkout main`.
+2. `git pull` to make sure you haven’t missed any last-minute commits. After this point, nothing else is making it into this version.
+3. `npm test` to ensure that all tests pass locally. Note that the local env must be ready (see [`circles-docker` repository](https://github.com/CirclesUBI/circles-docker)).
+4. `git push` and verify all tests pass on all CI services.
+5. Read the git history since the last release, for example via `git --no-pager log --oneline --no-decorate v2.10.10^..origin/main` (replace `v2.10.10` with the last published version).
+6. Create a git and npm tag based on [semantic versioning](https://semver.org/) using `npm version [major | minor | patch]`.
+7. `git push origin main --tags` to push the tag to GitHub.
+8. `git push origin main` to push the automatic `package.json` change after creating the tag.
+9. [Create](https://github.com/CirclesUBI/circles-payment-api/releases/new) a new release on GitHub, select the tag you've just pushed under _"Tag version"_ and use the same for the _"Release title"_. For the _"Describe this release"_ field, it can be autogenerated.