Installation on OS X requires two extra packages for frame reading [docs]

Created by: pmeyers279

The docs for install GWpy on OS X don't appear to complete if you want to read .gwf frames.

To get frame reading to work properly for GWpy on OS X I also had to run

port install py27-lal py27-lalframe

and then make sure to add these explicitly to my python path in the case where I was working in a virtualenv.

This was in addition to the rest of the current OS X installation instructions documented here.