Issue description

Certain packages will have a extras_require in setup.py.

Currently I've advised people to follow the convention of adding an option like
with$extra_name following the name used for the feature in setup.py.

See mailman-rss for example. (https://github.com/kyamagu/mailman-rss/blob/master/setup.py#L45).

Is this something to document specifically for Python in nixpkgs?