docs(open_graph): add comment

themes/typing/scripts/open_graph_cloudinary.js

@@ -1,3 +1,11 @@
+/*
+* Modified from the hexo version,
+* https://github.com/hexojs/hexo/blob/master/lib/plugins/helper/open_graph.js
+* for compatibility with cloudinary.js
+* the <meta name="og:image"> now use
+* data-src attribute of <img>, instead of src
+*/
+
 'use strict'
 
 const urlFn = require('url')