Document what's in astropy_helpers vs. core

There needs to be a prominent explanation in the documentation about what is in `astropy_helpers` / APE4 vs. what's in the core package itself.  This will help both devs and users know where to go if they want to either fix problems or see if other people have the issue.

This may belong as a new section in `installation`, or alternatively just a quick note there with a link, and more in-depth discussion in the developer docs.

I consider this crucial for the 0.4 release, so I'm milestoning it as such.