gitter badge in the readme pointing to gitlabhq/gitlabhq

What does this MR do?

Add the gitter badge to the readme, pointing to the existing chatroom at gitlabhq/gitlabhq

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

To promote the usage of the gitter room of GitLab.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?