Add region tags for docs (GoogleCloudPlatform#274)

Author: carieshmarie

appengine/flexible/twilio/app.yaml

@@ -4,7 +4,9 @@ env: flex
 runtime_config:
   document_root: .
 
+# [START env_variables]
 env_variables:
     TWILIO_ACCOUNT_SID: your-account-sid
     TWILIO_AUTH_TOKEN: your-auth-token
-    TWILIO_NUMBER: your-twilio-number
+    TWILIO_NUMBER: your-twilio-number
+# [END env_variables]