Move sphinx-specific :doc: entry out of README.rst

[peterjc]

On all the releases to date, including 0.6.8, the README is treated as plain text on PyPI:
https://pypi.org/project/PyVCF/0.6.8/

This is likely because it fails validation as a standalone RST file due to a Sphinx specific extension,

$ pip install restructuredtext-lint
...
$ restructuredtext-lint README.rst 
ERROR README.rst:179 Unknown interpreted text role "doc".

This fragment also renders poorly on GitHub, showing a blue clickable but non-functional link with text:

 :doc:`FILTERS`

This pull request ensures that README.rst is a valid standalone RST file and will render cleanly on GitHub and hopefully also PyPI, while preserving the link when used as part of the main documentation's introduction.