encoding/json: add an example for RawMessage

RawMessage is useful and mildly non-obvious.
Given the frequency with which RawMessage questions
show up on golang-nuts, and get answered with an example,
I suspect adding an example to the docs might help.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/14190044

Committer: Andrew Gerrand <adg@golang.org>