Update doc to include latest PEP 513 and many linux uploads to Pypi

[pombredanne]

The current doc is outdated and states only that Linux wheels are not supported on Pypi.
They are now with the latest versions of Pip under some conditions thanks to https://github.com/pypa/manylinux

[brettcannon]

This is specifically mentioned in https://packaging.python.org/distributing/#platform-wheels in a note.

[edmorley]

It's not clear to me whether that note is now out of date, or whether PyPI doesn't yet support manylinux (and that it was just that support was added to pip so far).

[ncoghlan]

The note is out of date, we just missed fixing it when support was enabled on PyPI. See numpy for example: https://pypi.python.org/pypi/numpy

[ncoghlan]

094aad1 at least fixes the note @brettcannon referenced. (Unfortunately, I mentioned the wrong issue in the commit message)

[edmorley]

Many thanks :-)

[edmorley]

Whilst the docs now mention manylinux1 after #283, it would be great if it was easier to find example projects that have a working build/upload setup eg using Travis. I found this example project, but I wonder if there are any other real-world examples too?
https://github.com/pypa/python-manylinux-demo