adding instructions for submitting pull requests. make them against -wip and not against master. all pull requests against master will be ignored

build/_includes/contribute.html

@@ -2,7 +2,8 @@
   <h2 class="page-header">GitHub Project</h2>
   <p class="lead">
     Found a bug? Want to request a new icon? Want to see what's up next for the project?
-    <a href="{{ site.font-awesome.github.url }}/issues">Submit an issue</a> on the
+    <a href="{{ site.font-awesome.github.url }}/issues">Submit an issue</a> or a
+    <a href="{{ site.font-awesome.github.url }}/pulls">pull request</a> on the
     <a href="{{ site.font-awesome.github.url }}">GitHub project</a>. Here are a few guidelines:
   </p>
   <ol>
@@ -16,8 +17,11 @@
         <li>Attach an image or two that represent the idea you're going for. Remember that font icons are a single color.</li>
       </ul>
     </li>
+    <li>
+      If you're submitting a pull request, make it on the latest -wip branch. All pull requests against master will be
+      ignored.
+    </li>
   </ol>
-
 </section>
 
 <section id="say-thanks">