Rename README.txt to README.rst to match the real ReST mark-up format

This is to prepare a move to GitHub for example.


Former-commit-id: 8c52508029525861cfc308003a2de78d39edcc5b
Improve the README of the project


Former-commit-id: c4174c001291559c92d480d093d5856c96ce8abb
Update installation URL in documentation


Former-commit-id: e44464be653dbb22ba6894ef8d4de02038fd3304
Merge branch 'p4a-own' into p4a


Former-commit-id: dc3d420b8193715272ace76884aab9e5f4aa07a1