Dev Docs: Briefly Describe JSON-RPC Format & bitcoin-cli

Briefly describe the JSON-RPC 1.0 format as used by Bitcoin Core and how
bitcoin-cli abstracts away some of the details.   Also link to the
Proper Money Handling wiki page.  (This as a vanguard PR preparing the
way for a larger overhaul of our RPC docs, including updating them for
0.10.)