Document issue with binary installers on Windows 7

I had a PyFITS user bring to me an odd problem he was having installing PyFITS on Windows 7 from the pre-built binary installer exe.  This does not necessarily affect all users on Windows 7.  In fact I didn't have this problem at all on my Windows 7 laptop.  But on further investigation it seems this problem *can* affect users depending on exactly how they installed Python and other system settings.  I describe the issue in more detail here: spacetelescope/PyFITS#26.

Eventually I'd like Astropy to distribute Wheel packages for Windows. But currently it's more feasible to distribute executable installers, so as long as we're doing that it's worth mentioning this issue in the installation docs.