Update the readme to make it more comprehensive (1046639)

This work fixes item 1 of Bugzilla bug [1046639](https://bugzilla.mozilla.org/show_bug.cgi?id=1046639).

This updates the readme with all the requested content. I iterated with jeads on it in-channel yesterday, and we are both reasonably happy with it. The only question in my mind is should .MD content have forced line breaks, or should the content line wrap? I am assuming the latter.

We have consciously chosen to use links with `_newtab` redirects to preserve the user's readme page.

Viewed on Windows in Github:
FF Release **31.0**
Chrome Latest Release **36.0.1985.125 m**

Adding @camd and @maurodoglio for visibility.