This allows godoc.org to link to the source code. I chose to link to
github.com instead of googlesource.com because their file browser is
better.
Details here:
864b1c0aba
Change-Id: I962b3fadc822e753245f52240937be5c7e158dc7
Reviewed-on: https://go-review.googlesource.com/2719
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>