docs(accordion): add is-open

Closes angular-ui#3230

Author: MattEvanoff

src/accordion/docs/readme.md

@@ -2,4 +2,9 @@ The **accordion directive** builds on top of the collapse directive to provide a
 
 We can control whether expanding an item will cause the other items to close, using the `close-others` attribute on accordion.
 
-The body of each accordion group is transcluded in to the body of the collapsible element.
+The body of each accordion group is transcluded in to the body of the collapsible element.
+
+### Accordion Settings ###
+
+  * `is-open` <i class="glyphicon glyphicon-eye-open"></i> (Defaults: false) :
+    Whether accordion group is open or closed.