diff --git a/millions.js b/millions.js
index 15e30bd..8902ff4 100644
--- a/millions.js
+++ b/millions.js
@@ -3,6 +3,7 @@
     // https://github.com/simpleanalytics/scripts/blob/master/src/default.js
     // https://docs.simpleanalytics.com/what-we-collect
     // https://docs.simpleanalytics.com/uniques
+    // https://christianheilmann.com/2015/12/25/detecting-adblock-without-an-extra-http-overhead/
 
     console.log('href', document.location.href);
     console.log('ref', document.referrer);