dashboard: add deployment comment to app.yaml

Also update default app and version to be correct.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5541044

misc/dashboard/app/app.yaml

-application: go-build
-version: 1
+# Update with
+#	google_appengine/appcfg.py [-V test-build] update .
+#
+# Using -V test-build will run as test-build.golang.org.
+
+application: golang-org
+version: build
 runtime: go
 api_version: 3