No doc about "how to release a version?"

There is currently no documentation about "how to release a new Batsim version?".

In short:

1. Select a new version name (following semantic versioning rules)
2. Update changelog (do not forget to add a link for the diff)
3. Update hardcoded version in CMakeLists.txt (used by out-of-git builds to determine batsim --version)
4. Tag -a the version (put changelog of released version in long description)
5. Push master and the tag on the CI server