Documentation for Database Adapter Interface

[curran]

From a discussion in #131 (comment) , it would be useful to have at least brief documentation on the interface that a database backend adapter must implement, and why.

Currently, to implement a new database adapter, it looks like the only way is to somewhat blindly port the code from one of the existing adapters:

• sharedb-mongo
• sharedb-mingo-memory
• sharedb-postgres
• sharedb-mysql

[WinMinTun]

One more DB adapter for Redis, you can check my library https://github.com/WinMinTun/sharedb-redis . It has also persistence option. I believe it is quite stable, feel free to let me know anything.