Document migration of outline button class in v4

See twbs#17719

Author: jorrit

docs/migration.md

@@ -100,6 +100,7 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 - Dropped the `.btn-xs` class entirely as `.btn-sm` is proportionally much smaller than v3's.
 - The [stateful button](http://getbootstrap.com/javascript/#buttons-methods) feature of the `button.js` jQuery plugin has been dropped. This includes the `$().button(string)` and `$().button('reset')` methods. We advise using a tiny bit of custom JavaScript instead, which will have the benefit of behaving exactly the way you want it to.
   - Note that the other features of the plugin (button checkboxes, button radios, single-toggle buttons) have been retained in v4.
+- Renamed `.btn-*-outline` to `.btn-outline-*`.
 
 ### Button group