Update robotframework readme file.

This PR makes an initial attempt at updating the `robotframework` readme file in light of trying to install the framework locally. It's intended as a discussion PR to some extent.

It attempts to do a few things:
+ order the sections a little better,
+ clarify some of the pre-requisites including a few foibles,
+ remove some more advanced stuff that affects readability,
+ explain all the various build options more fully.

There are a few things still to resolve:
+ are we sure about versions? It worked fine on FF27 for instance.
+ should we really recommend an RC version of Jython?
+ could we cut down on repetition (say of result locations)
+ should most of this really be moved to the website!?