Add link to the survey at the bottom of the page

Summary:
![simulator screen shot may 11 2016 3 03 14 pm](https://cloud.githubusercontent.com/assets/197597/15198889/3374abe6-178f-11e6-95e5-378004ea45aa.png)

<img width="1333" alt="screen shot 2016-05-11 at 3 03 25 pm" src="https://cloud.githubusercontent.com/assets/197597/15198886/2e545a8a-178f-11e6-8fd1-975ca700e49b.png">
Closes https://github.com/facebook/react-native/pull/7524

Reviewed By: JoelMarcey

Differential Revision: D3293684

Pulled By: vjeux

fbshipit-source-id: c0e7d138040eae49c6b7f7398de8d50f0ed2f564

website/layout/DocsLayout.js

@@ -48,6 +48,15 @@ var DocsLayout = React.createClass({
               {metadata.previous && <a className="docs-prev" href={'docs/' + metadata.previous + '.html#content'}>&larr; Prev</a>}
               {metadata.next && <a className="docs-next" href={'docs/' + metadata.next + '.html#content'}>Next &rarr;</a>}
             </div>
+            <div className="survey">
+              <div className="survey-image" />
+              <p>
+                We are planning improvements to the React Native documentation. Your responses to this short survey will go a long way in helping us provide valuable content. Thank you!
+              </p>
+              <center>
+                <a className="button" href="https://www.facebook.com/survey?oid=681969738611332">Take Survey</a>
+              </center>
+            </div>
           </div>
         </section>
       </Site>

website/src/react-native/css/react-native.css

@@ -1256,12 +1256,48 @@ div[data-twttr-id] iframe {
   background-color: hsl(198, 100%, 94%);
 }
 
+.survey {
+  padding: 10px;
+  padding-top: 0;
+  background-color: rgba(5, 165, 209, 0.05);
+  border: 2px solid #05A5D1;
+  position: relative;
+}
+
+.survey-image {
+  position: absolute;
+  top: 10px;
+  height: 83px;
+  width: 120px;
+  background-image: url('../img/survey.png');
+  background-size: 272px 198px;
+  background-position: -8px -8px;
+}
+
+.survey p {
+  margin-left: 130px;
+}
+
+.survey .button {
+  font-size: 24px;
+  background: #05A5D1;
+  color: #fafafa;
+}
+
 @media only screen
   and (max-device-width: 1024px) {
   #content {
     display: inline;
   }
 
+  .survey-image {
+    position: static;
+    margin: 10px auto;
+  }
+  .survey p {
+    margin-left: 0px;
+  }
+
   .container {
     min-width: 0;
     overflow: auto;