Drop README.pyramid.rst file
It has been kept since 3.24 when we merged pyramid-cubicweb and
cubicweb-pyramid repositories into cubicweb's one waiting for someone to move
the information in some canonical place but this never happened and this
information is probably not so useful or already existing elsewhere. So just
drop the file.
Refer to cubicweb.readthedocs.io in README
docs.cubicweb.org contains outdated documentation at the moment (and for quite
some time); until it gets update, refer to the up-to-date documentation on
readthedocs.