Create MR release template

Closes: #345

.gitlab/issue_templates/release.md

+# Release checklist:
+
+- [ ] update/review documentation and make sure it matches the current state
+  - Update how to checkout the latest stable tag
+- [ ] update [CHANGELOG.md](https://keepachangelog.com/en/1.0.0/)
+  - Include `Known issues`
+- [ ] update the version number in the `VERSION` file
+- [ ] commit
+- [ ] create signed tag (`git tag -s 0.2.0 -m'Release 0.2.0'`)
+- [ ] Push to MR, including tag