update Readme to include details on extending Store implementation from store.js

As noted in a comment on #97, developers who want to write a Store implementation either need to:
1. extend their Store from express-session's store.js, which typically means requiring express-session as a dependency
2. implement their own version of the functionality in store.js, session.js, and cookie.js (or copy the files from express-session)

This is because express-session requires the following methods on the Store in order to work:
- regenerate()
- load()
- createSession()

These are available in store.js, but `createSession` itself requires both session.js and cookie.js.

The documentation should be updated to provide guidance on these facts.