doc: add README
doc: add README

The README will appear on go.googlesource.com/scratch and show people
how to get started contributing to this repository.

Change-Id: I4b134071cda1c00e22ca53fcc233d9f0245930d2
Reviewed-on: https://go-review.googlesource.com/49450
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/scratch