Add security definition region tag (GoogleCloudPlatform#817)

Author: llatif

endpoints/getting-started/openapi.yaml

@@ -87,13 +87,14 @@ definitions:
         type: "string"
       email:
         type: "string"
-
+# [START securityDef]
 securityDefinitions:
   # This section configures basic authentication with an API key.
   api_key:
     type: "apiKey"
     name: "key"
     in: "query"
+# [END securityDef]
   # This section configures authentication using Google API Service Accounts
   # to sign a json web token. This is mostly used for server-to-server
   # communication.