devapp: update README to suggest 'gcloud auth configure-docker'
devapp: update README to suggest 'gcloud auth configure-docker'

Without this step, 'make push-staging' fails for me.

Change-Id: I4563775762671e39dc6a2cb2c5def403e908dfc7
Reviewed-on: https://go-review.googlesource.com/c/build/+/171359
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build