Document GitLab version compatibility

Problem to solve

As a potential user of glab, I want to know if I can use it with a self-hosted instance on an older GitLab version.

Proposal

Document somewhere (preferably the README I guess) the minimum version of GitLab that is expected for glab to work.

Further details

I tried glab with a GitLab Community Edition 9.5.10 instance, and glab mr list segfaults because the references attribute in the response does not exists in that version, so it crashes at this line:

https://gitlab.com/gitlab-org/cli/blob/7059cb3dfaa2491b6f285cb593e73fd9497c2c5e/commands/mr/mrutils/mrutils.go#L113.

Reading the "Version history" of the Merge request API endpoint here and the linked MR, I'm guessing the tool needs GitLab >= 12.7.0, at least for some features ?

Links / references