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
To enable S3 storage, set the following values in your lms.auth.json file, or preferably in your additional yaml overrides in your edx/configuration setup.

```javascript
    "AWS_ACCESS_KEY_ID": "your bucket AWS access key ID",
    "AWS_SECRET_ACCESS_KEY": "Your bucket AWS access key secret",
    "AWS_STORAGE_BUCKET_NAME": "Your upload bucket name",
```