godoc: add dummy playground.js to silence godoc warning at start-up
godoc: add dummy playground.js to silence godoc warning at start-up

Fixes issue 2173.

R=r, rsc
CC=golang-dev
http://codereview.appspot.com/4941044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang