chore(button): add known issue for tooltip/popover

* add known issue to button docs indicating a CSS issue when using a tooltip or
  a popover.

Closes #5890

Author: icfantv

src/buttons/docs/readme.md

@@ -44,3 +44,7 @@ With the buttons directive, we can make a group of buttons behave like a set of
 * `toggleEvent`
   _(Default: `click`)_ -
   Event used to toggle the buttons.
+
+### Known issues
+
+To use tooltips or popovers on elements within a `btn-group`, set the tooltip/popover `appendToBody` option to `true`. This is due to Bootstrap CSS styling. See [here](http://getbootstrap.com/components/#btn-groups) for more information.