Document difference to bitcoin core

[benma]

I can't find any information on what this fork implements exactly over bitcoin core, apart from "additional indexing". It would be nice to extend the README with detailed specification of the API calls added.

[lclc]

The additional features (more indexing and RPC calls for accessing those) are documented here: https://bitcore.io/guides/bitcoin

But would probably be good to have the documentation in this repo.