Add release guide (#186)

.github/PULL_REQUEST_TEMPLATE/release.md

@@ -0,0 +1,18 @@
+## Workflow to release a new version `vX.Y.Z`
+This is a checklist for a new release.
+It assumes a - [ ] Push and open PR (base onto `master`). Also push tag: `git push origin --tag`
+
+- [ ] Create new branch `vX.Y.Z` from `dev`
+- [ ] Bump version in `CADETProcess/__init__.py`
+- [ ] Bump version in `.zenodo.json` and update authors
+- [ ] Add release notes in `docs/source/release_notes/vX.Y.Z.md`
+  - [ ] General description
+  - [ ] Deprecations / other changes
+  - [ ] Closed Issues / PRs
+  - [ ] Add entry in `index.md`
+- [ ] When ready, rebase again onto `dev` (in case changes were made)
+- [ ] Commit with message `vX.Y.Z`
+- [ ] Add tag (`git tag 'vX.Y.Z'`) and push tag: (`git push origin --tag`)
+- [ ] Merge into master
+- [ ] Make release on GitHub using tag and release notes.
+- [ ] Check that workflows automatically publish to PyPI and [https://cadet-process.readthedocs.io/]