[Rivet] Updated Rivet release procedure notes

[Andy Buckley]

FYI, I've updated the mega-list of stuff to do when making a Rivet
release, for the few of us who have to do that:

https://rivet.hepforge.org/trac/wiki/ReleaseProcedure

Fortunately the faffiest bits are automated (phew)

-- 
Dr Andy Buckley, SUPA Advanced Research Fellow
Particle Physics Expt Group, University of Edinburgh

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.