Add links to subscribe to mailing list.

README.md

@@ -94,6 +94,11 @@ Join our mailing list:
 
   dev-pdf-js@lists.mozilla.org
   
+Subscribe either using lists.mozilla.org or Google Groups: 
+  
+  https://lists.mozilla.org/listinfo/dev-pdf-js
+  https://groups.google.com/group/mozilla.dev.pdf-js/topics
+
 Talk to us on IRC:
 
   #pdfjs on irc.mozilla.org