readme: removed flattr button as it does not support group repos

Linus Olsson:
Sorry but project pages on github can't be flattred. All our connections
are built on us asking the service API who the owner is. Github project
does not have an owner. So we can't know who should have the money :(
legal: re-wrote copying.md, added 3rd-party licenses

added detailed guidelines to copying.md.
Merge branch 'master' of github.com:SFTtech/openage
doc: fixed markdown in copying.md
repo: added .mailmap

git shortlog now mirrors the names and email addresses in copying.md
Move finding python into separate CMake module