Pdpinch/documentation updates

WIP. Not ready to merge yet. 

@cgee or @bdero can you review the installation instructions, esp. part 3:

https://github.com/mitodl/edx-sga/blob/pdpinch/documentation-updates/README.rst#installation
Just to lower the bar a little, we might want to specify that it needs to be added to `/edx/app/edxapp/edx-platform/requirements/edx/github.txt`, and maybe explicitly say that `-e git+https://github.com/mitodl/edx-sga@release#egg=edx-sga` is what needs to be added.

Maybe a short note being more specific about production would be good, i.e., explaining that you should do this on a branch you've made in a forked edx-platform repo for your org.