docs(datepicker): add is-open documentation

- Add documentation for `is-open` binding for the datepicker

Closes #4055
Fixes #3939

Author: wesleycho

Diff for: src/datepicker/docs/readme.md

@@ -118,6 +118,9 @@ Specific settings for the `datepicker-popup`, that can globally configured throu
  * `datepicker-append-to-body`
   _(Default: false)_:
   Append the datepicker popup element to `body`, rather than inserting after `datepicker-popup`. For global configuration, use `datepickerPopupConfig.appendToBody`.
+ * `is-open` <i class="glyphicon glyphicon-eye-open"></i>
+  _(Default: false)_:
+  Whether to show the datepicker.
 
 ### Keyboard Support ###