docs(accordion): add event support documentation

- Add documentation for collapse events available for accordion

Author: svegliar

src/accordion/docs/readme.md

@@ -4,6 +4,26 @@ The body of each accordion group is transcluded into the body of the collapsible
 
 ### uib-accordion settings
 
+* `collapsed()`
+  <small class="badge">$</small> -
+  An optional expression called after the element finished collapsing.
+
+* `collapsing()`
+  <small class="badge">$</small> -
+  An optional expression called before the element begins collapsing.
+  If the expression returns a promise, animation won't start until the promise resolves.
+  If the returned promise is rejected, collapsing will be cancelled.
+
+* `expanded()`
+  <small class="badge">$</small> -
+  An optional expression called after the element finished expanding.
+
+* `expanding()`
+  <small class="badge">$</small> -
+  An optional expression called before the element begins expanding.
+  If the expression returns a promise, animation won't start until the promise resolves.
+  If the returned promise is rejected, expanding will be cancelled.
+
 * `close-others`
   <small class="badge">$</small>
   <small class="badge">C</small>