Add 'Getting Started' message to README.rst

Quoting gsilvis (2014-06-24 09:47:14)
> > +You only need to do this once.
> 
>    Which two lines are you asking to merge? Github isn't showing me
>    exactly what line this is a comment on.

You say "you only need to do this once" twice, albeit not for *exactly*
the same thing. Still, it feels a little jumbled.