Replace ancient README files with a not quite as ancient one

I merged most of the three separate README files into a single one. I
also assumed that people trying to build OpenClonk from source have a
rough idea of how their build system works, so I removed the tutorial
from there. People who are completely new to this should consult the
wiki or ask on IRC.
Remove autotools files

Building OpenClonk with GNU autotools has been broken since early August
at least (9ceb692). Since nobody complained so far, I'm assuming nobody
has used it recently, and opted to remove the broken code.