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
Instead of deleting the var to set it back to default right after it is set, I would suggest telling them to replace the value of `DEFAULT_FILE_STORAGE` with `django.core.files.storage.FileSystemStorage`.  The MEDIA_ROOT hash is still required and correct