Update top-level README

[vchudnov-g]

The top-level README.rst is very sparse, and it says that the repository "primarily exists to facilitate experimentation", which is no longer true.

We should update the file to say this library is used for official client libraries. Moreover, we should consider whether to add slightly more information to the README.rst, even though it seems proper to refer people to ReadTheDocs for the bulk of the documentation. (See also issue #1390 about auditing that documentation, too.)