docs: Add typing-indicators.md.

static/js/typing.js

@@ -4,6 +4,8 @@ var exports = {};
 // This module handles the outbound side of typing indicators.
 // We detect changes in the compose box and notify the server
 // when we are typing.  For the inbound side see typing_events.js.
+//
+// See docs/typing-indicators.md for details on typing indicators.
 
 function send_typing_notification_ajax(recipients, operation) {
     channel.post({