diff --git a/README.services.md b/README.services.md
index 87d675a..b039836 100644
--- a/README.services.md
+++ b/README.services.md
@@ -20,3 +20,5 @@ Take a look at Django signals:
 
 * pre_save
 * post_save
+
+Ref: https://www.dropbox.com/developers/webhooks/tutorial