Add nbviewer link to playground

Author: aleneum

examples/Playground.ipynb

@@ -8,6 +8,8 @@
     "\n",
     "Make sure to read the [documentation](https://github.com/pytransitions/transitions#table-of-contents) first.\n",
     "\n",
+    "Links won't work on Github but should work with [nbviewer](https://nbviewer.org/github/pytransitions/transitions/blob/master/examples/Playground.ipynb).\n",
+    "\n",
     "* [Rescue those kittens!](#Rescue-those-kittens!)\n",
     "* [Too much coffee with my hierarchical state machines!](#Too-much-coffee-with-my-hierarchical-state-machines!)\n",
     "* [Very asynchronous dancing](#Very-asynchronous-dancing)\n",