Document how hook call order is affected by plugin registration

[nicoddemus]

As discussed in #62, we should explicitly document how hook call order is done regarding how plugins are registered.

From the top of my head, hooks are called in this order:

1. setuptools plugins (in arbitrary order, except those marked with try_first and try_last);
2. core plugins;

[astrojuanlu]

setuptools plugins (in arbitrary order

I know this is an ancient issue but I don't think this fact was sufficiently documented in gh-85.