Update doc to reflect us-west-1 support
Update doc to reflect us-west-1 support

https://github.com/spinnaker/rosco/pull/86
jpancoast-kenzan/spinnaker-terraform,kenzanlabs/spinnaker-terraform,jpancoast-kenzan/spinnaker-terraform,kenzanlabs/spinnaker-terraform,kenzanlabs/spinnaker-terraform,jpancoast-kenzan/spinnaker-terraform