Update docs

README.md

@@ -9,10 +9,13 @@ With this, you can set up a listener in one window, have it wait for a post mess
 and gracefully handle any errors that crop up. You can also set a timeout, to be sure that the other window is responding to you,
 and fail gracefully if it does not.
 
+This also allows cross-domain post messages between two different windows (not just popups) in IE9+.
+
 ## Features
 
 - Request/response pattern (avoids sending fire-and-forget messages back and forth)
 - Don't worry about serialization, just send javascript objects
+- Handles all of the corner cases for IE9+, which is normally not able to send cross-domain post messages between two different windows, only iframes
 - Handle error cases gracefully
   - The user closed the window you're trying to message
   - You sent a message the other window wasn't expecting
@@ -56,4 +59,46 @@ postRobot.once('init', function(err, data, callback) {
 postRobot.send(window, 'init', function(err, data) {
     console.log('init done');
 });
-```
+```
+
+## IE9+
+
+In order to use post-robot in IE9+ between two different windows on different domains (like a parent window and a popup)
+you will need to set up an invisible bridge in an iframe on your parent page:
+
+```
++---------------------+
+| Parent xx.com       |
+|                     |      +--------------+
+|  +---------------+  |      | Popup yy.com |
+|  | Bridge yy.com |  |      |              |
+|  |               |  |      |              |
+|  |               |  |      |              |
+|  |               |  |      |              |
+|  |               |  |      |              |
+|  |               |  |      |              |
+|  |               |  |      +--------------+
+|  +---------------+  |
+|                     |
++---------------------+
+```
+
+Doing this is pretty simple:
+
+1. Create a bridge path, for example `http://yy.com/bridge.html`, and include post-robot:
+
+```html
+<script src="http://xx.com/js/post-robot.js"></script>
+```
+
+2. In the parent page, `xx.com`, include the following javascript:
+
+```html
+<script>
+    postRobot.openBridge('http://yy.com/bridge.html');
+</script>
+```
+
+Now `Parent xx.com` and `Popup yy.com` can communicate freely using post-robot in IE.
+
+This can even be done in reverse -- for example, `Popup yy.com` can include `Bridge xx.com` if that suits your use cases better.