Provide more documentation in README

[vendillah]

I suggest adding a few more points of information to the README:

• Even though I'm sure I can manage to find out how to custom install this extension; would you perhaps provide some installation info? I'm sure it will help out other people that are looking for such an extension.
• This is a gnome extension, right? It isn't mentioned in the README either. Also, mentioning the last (or just any) version of Gnome that is compatible with it is useful knowledge! Well, if this is anything else, too bad for me, but at least other people will regard it as such! :-)

It would be greatly appreciated.