Add .rst copy of README

The README is already in reST format, but Github doesn't render it because it has the .txt extension. Giving it a .rst extension means the GH project page shows it rendered.

I've left the .txt copy for now, because [this doc](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt) links to it. With this merged, that can be updated and the .txt copy removed.