OSDOCS#4688: Added a xref to Google docs detailing supported machine type by region and zone

Author: mjpytlak

modules/installation-gcp-regions.adoc

@@ -47,3 +47,8 @@ regions:
 * `us-west2` (Los Angeles, California, USA)
 * `us-west3` (Salt Lake City, Utah, USA)
 * `us-west4` (Las Vegas, Nevada, USA)
+
+[NOTE]
+====
+To determine which machine type instances are available by region and zone, see the Google link:https://cloud.google.com/compute/docs/regions-zones#available[documentation].
+====