Open source setup script + basic documentation

Provide `setup-github-mirror.sh` as tool to automatically set up GitLab
projects to be used for `mirror.sh` to keep upstream work around
locally.

A basic README should provide enough information for people to setup the
project successfully in their environment.

What `./setup-github-mirror.sh` does:
 - Collect information from GitHub about the repository to mirror
 - Create a subgroup in the `github-mirror` group with the github
username of the repository owner
 - Create the project with the github repository name within the created
subgroup
 - Configure the project to use the github avatar as project avatar,
disable issues, merge requests, CI/CD integration, wiki, and snippets,
and cofigure the project description to the repository description from
GitHub along with a reference to the upstream repository
 - Enable the deployment key of the mirror bot for the project and grant
it write access
 - Provide a ready to use `./mirror.sh` instructions

The goal of `setup-github-mirror.sh` is to take away all manual steps
that are required to setup a repository that can be used with
`mirror.sh`. It's not intended to be used in an automated way in first
place, but can definitely be used by other scripts that scrape
organisation or user metadata and mirror those entirely.

The script is setup to be run more than once without making unneeded
changes. It'll always result in the configuration mentioned above and is
therefore idempotent.