Adds GitHub link to docs.

I know this sounds a little bit ridiculous, however I struggle to find the link to
the repo on GitHub most of the time. This commit adds a little icon along with the
Twitter and co icons.

book.json

@@ -3,11 +3,14 @@
   "structure": {
     "summary": "docs/README.md"
   },
-  "plugins": ["edit-link", "prism", "-highlight"],
+  "plugins": ["edit-link", "prism", "-highlight", "github"],
   "pluginsConfig": {
     "edit-link": {
       "base": "https://github.com/rackt/redux/tree/master",
       "label": "Edit This Page"
+    },
+    "github": {
+      "url": "https://github.com/rackt/redux/"
     }
   }
 }