add release instructions

RELEASE.md

@@ -0,0 +1,22 @@
+# Releasing
+
+Each push to GitHub triggers a new GitHub Actions build using the maven-ci workflow.
+
+To release, simply create a tag:
+
+```
+git tag -a v1.0.0 -m "release v1.0.0"
+git push origin v1.0.0
+```
+
+This triggers a new GitHub Actions build (maven-release workflow) using the tag as version number and creates a draft release with the release artifact attached.
+On the releases GitHub page, you can edit the release notes, set the pre-release checkbox if applicable, and publish the release.
+
+If the build fails because the staging repository is down, you can restart the build by re-tagging it:
+
+```
+git tag -d v1.0.0
+git push --delete origin v1.0.0
+git tag -a v1.0.0 -m "release v1.0.0" 
+git push origin v1.0.0
+```