doc: add README
doc: add README

Change-Id: Ia281ebc576c92d4a1ce79f5e08984ca5f00390f7
Reviewed-on: https://go-review.googlesource.com/14811
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

gophergala2016/playground-plus,golang/playground,gophergala2016/playground-plus,gophergala2016/playground-plus,golang/playground,golang/playground