Update the README.rst

We currently have an extremely minimal `README.rst` in this repo, which essentially redirects the user to (stale?) info in https://googleapis.dev/python/gapic-generator-python/latest/. Moreover, the `README` says this repo "primarily exists to facilitate experimentation". We're way past that point!

We should update the `README` and have it reflect more of the structure used for the other generators' `README`s. If we depend on linking to some of that content (eg googleapis.dev) we should ensure that information is current as well.