Documentation about pre/post hooks

[astrofrog]

While documenting astropy-helpers in #416 I forgot to mention the pre/post hook functionality for setup_package.py - basically:

Look through setup_package.py modules for functions with names like
``pre_<command_name>_hook`` and ``post_<command_name>_hook`` where
``<command_name>`` is the name of a ``setup.py`` command (e.g. build_ext).

We use this in astropy._erfa and astropy.modeling to run the preprocessor on source code.

[astrofrog]

At the same time we should probably investigate if there is a more standard way to do this.

[astrofrog]

I'm going to close this - with APE 17 merged I don't think we want to advertise features in astropy-helpers that were previously undocumented, and which will no longer be valid post-APE 17.