Improving documentation

This is an attempt to make content-store easier to understand from a standing start. The main focus has been:

* Consistent use of language and terminology
* Updating/deleting misleading or out of date information
* Writing the README so it makes sense to someone new to content-store
This URL isn't quite correct.  The `base_path` has a leading `/`, so this will result in a double `/` in the path.