Document "ripplerpc": "3.0" option

[intelliot]

This is a reminder to document the "ripplerpc": "3.0" option if/when the following PR gets merged.

XRPLF/rippled#4143

[mDuo13]

Also document any general differences that "ripplerpc": "2.0" applies as well. XRPLF/rippled#1907
(I'm currently not sure what changes, if any, are applied as a result of passing either version)

[mDuo13]

The 2.0 stuff is related to supporting JSON-RPC 2.0 (differences from 1.0, generally, not specific to rippled, are here: http://www.simple-is-better.org/rpc/#differences-between-1-0-and-2-0 ).

The 3.0 stuff mainly lets the response use non-200 HTTP status codes for errors, which causes the JSON-RPC API to behave a bit more like a RESTful API.