Add docs survey link to site header

Summary:
**Test plan**

Tested locally to see link in header bar and that it takes you to survey.

![screenshot 2016-05-11 08 21 11](https://cloud.githubusercontent.com/assets/3757713/15186299/593fd4f6-1751-11e6-847f-ea73ce4e2809.png)
Closes https://github.com/facebook/react-native/pull/7514

Differential Revision: D3287859

fbshipit-source-id: 48dfdbae74622330f192d5763c5e47025f1887b1

website/core/HeaderLinks.js

@@ -22,14 +22,23 @@ var HeaderLinks = React.createClass({
   linksExternal: [
     {section: 'github', href: 'https://github.com/facebook/react-native', text: 'GitHub'},
     {section: 'react', href: 'http://facebook.github.io/react', text: 'React'},
+    {
+      section: 'survey',
+      href: 'https://www.facebook.com/survey?oid=681969738611332',
+      text: 'Take Our Docs Survey'
+    }
   ],
 
   makeLinks: function(links) {
+    var surveyStyle = {
+      color: '#2AFF48',
+    };
     return links.map(function(link) {
       return (
         <li key={link.section}>
           <a
             href={link.href}
+            style={link.section === 'survey' ? surveyStyle : null}
             className={link.section === this.props.section ? 'active' : ''}>
             {link.text}
           </a>