cherry-pick 5024b24348f90220747e3e04c0d1fdf495a38d9c
README: + new notes about the diff. between 2 branches

The two branches :core and :master are now very different. I do not
now why this happens. But this is very true. We now do not want to
develop :core features on :master, nor to develop :users stuff on
the :core -- that should be used for Ruby scripts