Expand the README.md with more details
The README.md used to have a bit more detail about gitlab-shell, let's return some of the detail including:
- A description of the project. We can use https://docs.gitlab.com/ee/development/gitlab_shell/ as a base, but that also needs to be updated to also refer to
gitlab-sshdas an option. - How to build.
- How to test.
- How to contribute.
- LICENSE information.
Edited by Ash McKenzie