Add link for help on creating a custom transition

Author: zeckdude

docs/src/pages/components/transitions/transitions.md

@@ -88,3 +88,5 @@ It should respect the following conditions:
 - Call the `onEnter` callback prop when the enter transition starts.
 - Call the `onExited` callback prop when the exit transition is completed.
 These two callbacks allow to unmount the children when in a closed state and fully transitioned.
+
+For more information on creating a custom transition, visit the [React Transition Group Transition docs](http://reactcommunity.org/react-transition-group/transition).