Revise developer documentation
requested to merge github/fork/t-b/revamp-installation-and-contribution-instructions into tango-9-lts
Remove CONTRIBUTE.md as it is superfluous.
Rework INSTALL.md with the follwoing goals:
- Describe all CMake variables in a easy-to-read manner
- Explain how cross-compiling tango works
- Explain how to compile the tango dependencies
@bourtemb @mliszcz Sorry for rushing this now.
But I would need that for tuesday. I've also given you write access to the forked repo. So for bigger changes please push them as fixup commits. This is way more convenient than github comments.