Add notes to refer to docs when modifying the JS files

resources/js/admin/app.js

@@ -1,8 +1,14 @@
+/**
+ * Before you edit these, read the documentation on how these files are compiled:
+ * https://docs.phpvms.net/customize/building-assets
+ *
+ * Edits here don't take place until you compile these assets and then upload them.
+ */
+
 /**
  * Admin stuff needed
  */
 
-
 import airport_lookup from './airport_lookup';
 import calculate_distance from './calculate_distance';