Merge branch '116-explain-the-release-tagging-convention' into 'master'

Resolve "Explain the release/tagging convention"

Closes #116

See merge request https://gitlab.com/gitlab-org/terraform-images/-/merge_requests/231



Merged-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Co-authored-by: Timo Furrer <tfurrer@gitlab.com>