extras_require not documented in distributing-packages.rst

[zmwangx]

extras_require is documented at setuptools.readthedocs.io but not in distributing-packages.rst.

[theacodes]

This is intentional, as the distributing package tutorial is pretty minimal. I want to address this when I write the new tutorials (#203 et al), I just haven't quite found time.

In the meantime, I could see a separate guide on this.

Closing for now. If you want to see a specific guide we'd need to figure out what to stick in it.

[zmwangx]

Thanks for the explanation. I don't really need a specific guide — I just had trouble recalling the name extras_require yesterday, so I went to the guide, and realized it's not there. If it doesn't belong there in the first place, no big deal.