Add solution for haproxy issue (WhatsApp#103)

FAQ.md

@@ -42,4 +42,8 @@ the container's port 80 binds to my machine's 8081, which is what will be expose
 WhatsApp currently does **NOT** support anything besides TCP proxying. This is just copying the incoming bytes 
 to WhatsApp on the other end. So we don't support running through any intermediary that is a HTTP proxy.
 
-You are free to run your own pure TCP proxy as you see fit however, as long as it forwards to `g.whatsapp.net`. You aren't required to use this realization.
+You are free to run your own pure TCP proxy as you see fit however, as long as it forwards to `g.whatsapp.net`. You aren't required to use this realization.
+
+### (6) haproxy `cannot bind socket (Permission denied)`
+
+See https://github.com/docker-library/haproxy/issues/160 for possible solutions.