Install "react" and "react-dom" packages

Since Gatsby v2, React (1) is a peer dependency which allows users to
freely choose the version they'd like to use. Nord Docs will initially
be build with the currently latest stable version 16.6.3 (2).

References:
  (1) https://reactjs.org
  (2) https://www.npmjs.com/package/react/v/16.6.3

Relates to GH-24
Closes GH-33

package.json

@@ -73,6 +73,8 @@
     "gatsby-plugin-remove-trailing-slashes": "2.0.5",
     "gatsby-source-filesystem": "2.0.8",
     "gatsby-transformer-yaml": "2.1.5",
-    "nord": ">=0.2.1 <1.0.0"
+    "nord": ">=0.2.1 <1.0.0",
+    "react": "16.6.3",
+    "react-dom": "16.6.3"
   }
 }