Update README
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1868346
    
    - Explain what "provisional" means
    - Say what kinds of projects the macros handle
    - Tell people to BuidRequire python*-devel
    - Say %pyproject_buildrequires is needed for the other macros
    - Note that %pyproject_buildrequires generates `exit 11` messages
    - Note that most buildsystems have prepare-metadata-for-build-wheel