README: Update instructions on checking out code.
README: Update instructions on checking out code.

Change-Id: I64a2e107faaee09e78cf11a402de95f580fee308
Reviewed-on: https://go-review.googlesource.com/37407
Reviewed-by: Tuo Shan <e38988508c7dd9b59ee1f5735040284711960df8@google.com>

segmentio/gddo,segmentio/gddo,golang/gddo,golang/gddo,segmentio/gddo,golang/gddo,golang/gddo,segmentio/gddo