[docs] [issue18707] the readme should also talk about how to build doc.

[Madison May]

Madison May added the comment:

Good point, Éric.

It refers readers to the bug tracker, the Sphinx bug tracker, and docs at python.org in the `Contributing` section, but a link to the devguide would probably be more useful.  I've tweaked the patch a bit to reflect this.

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Added file: http://bugs.python.org/file31268/README_v3.diff

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