Documentation: Point to documentation in README and in PyPI listing

[jpgoldberg]

Neither the README.rst file for this repository nor the project information, as presented on https://pypi.org/project/PyNaCl/ (v1.5.0) give any indication that https://pynacl.readthedocs.io/en/latest/ exists.

I now see that that a Documentation URL is now in pyproject.toml for version 1.6.0.dev1, so I am hoping that that will be released soon, so I am just recommending that the README be updated.