Hide GCP install guide from index page until our image is added

doc/install/README.md

@@ -18,8 +18,6 @@ the hardware requirements.
   Useful for unsupported systems like *BSD. For an overview of the directory
   structure, read the [structure documentation](structure.md).
 - [Docker](https://docs.gitlab.com/omnibus/docker/) - Install GitLab using Docker.
-- [Installation on Google Cloud Platform](google_cloud_platform/index.md) - Install
-  GitLab on Google Cloud Platform using our official image.
 - Testing only! [DigitalOcean and Docker Machine](digitaloceandocker.md) -
   Quickly test any version of GitLab on DigitalOcean using Docker Machine.

doc/install/google_cloud_platform/index.md

@@ -2,6 +2,10 @@
 ![GCP landing page](img/gcp_landing.png)
+>**Important note:**
+GitLab has no official images in Google Cloud Platform yet. This guide serves
+as a template for when the GitLab VM will be available.
+
 The fastest way to get started on [Google Cloud Platform (GCP)][gcp] is through
 the [Google Cloud Launcher][launcher] program.