Add notes for CORS settings on Google Cloud Storage

docs/advanced_topics/deploying.rst

@@ -57,4 +57,24 @@ If you are also serving Wagtail's static files from remote storage (using Django
         'Access-Control-Allow-Origin': '*'
     }
 
+For Google Cloud Storage, create a ``cors.json`` configuration:
+
+.. code-block:: json
+
+	[
+	    {
+	      "origin": ["*"],
+	      "responseHeader": ["Content-Type"],
+	      "method": ["GET"],
+	      "maxAgeSeconds": 3600
+	    }
+	]
+
+Then, apply this CORS configuration to the storage bucket:
+
+.. code-block:: shell
+
+    gsutil cors set cors.json gs://$GS_BUCKET_NAME
+
+
 For other storage services, refer to your provider's documentation, or the documentation for the Django storage backend library you're using.