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

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old