Add brief notes to release process

RELEASE.md

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+# Making a release
+
+When the version changes, a new release should be cut. To do this, push a tag
+with the [valid SemVer][semver-checker] version number as the tag.
+It may also be useful to update documentation references at the same time.
+
+## Example
+
+For Bookstack version 23.01:
+
+```shell
+sed -i '' -e 's/22.11.1/23.1.0/g' *     # 22.11.1 was the previous version
+git commit -am "Update references to version 23.1.0" [-S]
+git tag [-s] -a 23.1.0 -m "Release version 23.01"
+git push --tags
+```
+
+The workflow will then build, test, push, and release this image.
+
+[semver-checker]: https://jubianchi.github.io/semver-check/