how to release documentation (#233)

RELEASE_howto.md

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+# How to do a HDM release
+
+## Prepare gemset
+
+```shell
+bundle config set --local path 'vendor/bundle'
+bundle install
+```
+
+## Merge pull requests
+
+1. check labels on issues and pull requests on github
+1. check mentions from pull requests to issues
+1. merge all desired pull requests
+
+## Create Release PR
+
+```shell
+export CHANGELOG_GITHUB_TOKEN="github_TOKEN"
+git switch -c release/X.Y.Z
+```
+
+edit `Rakefile`, search replace `future_release`
+
+```text
+    config.future_release = 'X.Y.Z'
+```
+
+Create Changelog
+
+```shell
+bundle exec rake changelog
+```
+
+check generated changlog
+
+Create release pull request
+
+```shell
+git add -A
+git commit -m 'Release vX.Y.Z'
+git push
+```
+
+Open pull request on GitHub, add `skip-changelog` label and merge
+
+## Set release tag
+
+```shell
+git switch main
+git pull -r
+git tag vX.Y.Z
+git push --tags
+```
+
+## Generate GitHub release
+
+Open [Releases - betadots/hdm](https://github.com/betadots/hdm/releases) in browser.
+
+Click on `Draft a new release`
+
+Set release title to tag name (X.Y.Z)
+
+Choose tag
+
+Click on `generate release notes`
+
+Publish release
+
+Done.