First draft of the omnibus-gitlab release process

doc/release.md

+# Omnibus-gitlab release process
+
+Our main goal is to make it clear which version of GitLab is in an omnibus package.
+
+## On your development machine
+
+- Pick a tag of GitLab to package (e.g. `v6.6.0`).
+- Create a release branch in omnibus-gitlab (e.g. `6-6-stable`).
+- Change [the gitlab-rails version in omnibus-gitlab] (e.g. `version "v6.6.0"`).
+- Commit the new version to the release branch
+
+```shell
+# Example:
+git commit -m 'Pin GitLab to v6.6.0' config/software/gitlab-rails.rb
+```
+
+- Create an annotated tag on omnibus-gitlab corresponding to the GitLab tag.
+  GitLab tag `v6.6.0` becomes omnibus-gitlab tag `6.6.0.omnibus`.
+
+```shell
+# Example:
+git tag -a 6.6.0.omnibus -m 'Pin GitLab to v6.6.0'
+```
+
+- Push the branch and the tag to the main repository.
+
+```shell
+# Example:
+git push origin 6-6-stable 6.6.0.omnibus
+```
+
+## On the build machines
+
+- Check out the release branch of omnibus-gitlab.
+
+```shell
+# Example
+git fetch
+git checkout 6-6-stable
+```
+
+- Check the version with `git describe`.
+
+```shell
+# Example
+git describe # Should start with 6.6.0.omnibus
+```
+
+- Build a package with version timestamps disabled.
+
+```shell
+# Example
+OMNIBUS_APPEND_TIMESTAMP=0 bin/omnibus build project gitlab
+```
+
+[the gitlab-rails version in omnibus-gitlab]: ../config/software/gitlab-rails.rb#L20