Updates the PyFITS FAQ to serve as the astropy.io.fits FAQ

Also removed a few questions mostly related to installation that are already covered generally by the Astropy installation docs, and made some small improvements to other existing answers, including improving links to the API docs.

This brings to mind that maybe it would be good to have a general Astropy FAQ.  This would be different from the "known issues" page which specifically addresses known bugs and shortcomings.  But a FAQ that addresses common usage questions both in specific submodules and across Astropy (how to deal with warnings, logging, config, etc.) might be handy.  Most *answers* could be short, and link to the relevant documentation.  But I think it should be possible to come up with a fair number of actual FAQs.
I'm not sure I understand your first point.

The second point about byteswapping is fair though--I completely neglected that--my bad.  When I change the benchmark to include byteswapping that does have a not insignificant slowdown, though it was still ultimately faster than fitsio (though by a slimmer margin).  Though you're right that the need to byteswap (among other things) hurts the overall effectiveness of mmap here.  This is one thing I've brought up in discussions over why I think FITS should adopt a convention to allow either byte order, though that idea was dismissed without any real good reason (FITS is not a network protocol...)

This was just my attempt to explain the discrepancies I did see--I'm not wrong that not using mmap still results in at least one additional copy.  

I would definitely welcome more thorough outside benchmarking though.  This was just a quick effort to have *something*, since I had been asked.