[Rivet] installation

[David Grellscheid]

Hi Frank,

> "It could be a nice tool, but unfortunately in its latest versions it 
> comes with so many dependencies that I cannot recommend to use it now.  
> It is just not matured enough and the feedback by the core developer was 
> less than helpful."

That is just plainly wrong when applied to _production_ public tarball
releases. If I told people that Sherpa was crap because half the time
when I take some random SVN version it doesn't even work, you would
rightly object.

Are there problems with released tarballs? If yes, what are they? I will
try to fix them.

Also, I would rewrite the Rivet webpages to make references to
development versions _much_ less prominent, to avoid confusion in future.

  David