Improve project description and add link to readthedocs

[slel]

The main page for this repo on GitHub,

https://github.com/aleaxit/gmpy

has this description

Automatically exported from code.google.com/p/gmpy

which could be replaced by a description of the project such as

Python module for multiple-precision arithmetic with GMP, MPIR, MPFR, MPC

or the same description as on PyPI:

GMP/MPIR, MPFR, and MPC interface to Python 2.6+ and 3.x

In addition, GitHub lets the project owner set a url for the project,
and it would be nice if this could be filled with the readthedocs url:

https://gmpy2.readthedocs.io/en/latest/

It was also suggested at #146 (comment)
to link to readthedocs from the PyPI page.

Finally, the readthedocs page could have links to the GitHub repo
and to the PyPI page.

[casevh]

Unfortunately, I'm not the project owner. A have sent him an email.

[aleaxit]

Done for this github repo -- Case, can you please do the changes to readthedocs and pypi?