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({

static/js/typing_data.js

@@ -1,6 +1,8 @@
 var typing_data = (function () {
 var exports = {};
 
+// See docs/typing-indicators.md for details on typing indicators.
+
 var typist_dct = new Dict();
 var inbound_timer_dict = new Dict();
 

static/js/typing_events.js

@@ -1,6 +1,8 @@
 var typing_events = (function () {
 var exports = {};
 
+// See docs/typing-indicators.md for details on typing indicators.
+
 // This code handles the inbound side of typing notifications.
 // When another user is typing, we process the events here.
 //

static/js/typing_status.js

@@ -2,6 +2,8 @@ var typing_status = (function () {
 
 var exports = {};
 
+// See docs/typing-indicators.md for details on typing indicators.
+
 // The following constants are tuned to work with
 // TYPING_STARTED_EXPIRY_PERIOD, which is what the other
 // users will use to time out our messages.  (Or us,