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* Updating/deleting misleading or out of date information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
I'm not sure this statement is quite correct.  The keys may sometimes represent the format of the linked item (as in these examples), but in other cases, they won't (eg related_items).