[issue13388] document hg commit hooks in the devguide

[Petri Lehtinen]

Petri Lehtinen <petri at digip.org> added the comment:

I'm not sure if it matters, but Sphinx documentation suggests using ^ as the underlining character for subsubsections.

Otherwise, looks good.

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nosy: +petri.lehtinen

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<http://bugs.python.org/issue13388>
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