Update README

[mrts]

README needs better explanations of the use case and better structuring.

1. Overview
2. Simple mirror
   1. Use case
   2. Setup
   3. Any caveats, e.g. deadlock?
3. GitLab mirror with satellite
   1. Use case: want to do code review and merge requests in GitLab, otherwise code in origin repo
   2. "Origin takes precedence"-policy - GitLab only pushes changed refs. Is this relevant for simple too?
   3. Setup
   4. Caveats
4. Deadlock caveat. Cron script needs to be in separate location from the script in post-receive hook, otherwise two runs of the same script will deadlock as follows:
   1. cron: call script
   2. script: create lock, push changes from origin to gitlab, wait for gitlab response
   3. gitlab: call script from post-receive hook
   4. script: wait for lock created by the instance run from cron
   5. ⇒ deadlock