message: [json-rpc] update the README
  Closes:
diff: # JSON-RPC
  -JSON-RPC service provides APIs for client applications to connect to Libra blockchain.
  +JSON-RPC service provides APIs for client applications to query the Libra blockchain.
  +
  +There are two types of clients, and thus two types of APIs:
  +
  +* verifying clients. These clients verify the cryptographic proofs accompanying responses from the server.
  +* non-verifying clients. Such clients completely trust the full node they query. If the server decides to lie about the data contained in a response, the client wouldn't know it. This mode is used when a client runs their own full node.
  +
  +## Usage and versioning
  +
  +Refer to the [specification](https://github.com/libra/libra/blob/master/json-rpc/json-rpc-spec.md).
  +
  +## Security Concerns
  +
  +It currently is exposed to 0.0.0.0 by default, so be aware of that.
  +
  +It also uses liberal CORS policies, so its API must not use any secrets (e.g. tokens, credentials, etc.)
  ## Test
  