include the version number in the header (facebook#8315)

Author: Kevin Lacker

docs/_includes/navigation.html

@@ -16,7 +16,7 @@
       </ul>
       <ul class="nav-site nav-site-external">
         <li><a href="https://github.com/facebook/react">GitHub</a></li>
-        <li><a href="https://facebook.github.io/react-native/">React Native</a></li>
+        <li><a href="https://github.com/facebook/react/releases">v{{site.react_version}}</a></li>
       </ul>
     </div>
   </div>