ARROW-2881: [Website] Add community tab to header, add link and callout to dev wiki

Also resolves ARROW-2804

Author: Wes McKinney <wesm+git@apache.org>

Closes #2320 from wesm/ARROW-2881 and squashes the following commits:

bd6d257c <Wes McKinney> Fix english
9c72510e <Wes McKinney> Add community tab to header, add link and callout to dev wiki
diff --git a/_config.yml b/_config.yml
index b9dd723..beb4021 100644
--- a/_config.yml
+++ b/_config.yml
@@ -35,6 +35,7 @@
   - asf-site
   - scripts
   - README.md
+  - vendor
 
 collections:
   docs:
diff --git a/_includes/header.html b/_includes/header.html
index a7428675..9e4644c 100644
--- a/_includes/header.html
+++ b/_includes/header.html
@@ -24,7 +24,16 @@
             <li><a href="{{ site.baseurl }}/release/">Releases</a></li>
             <li><a href="https://issues.apache.org/jira/browse/ARROW">Issue Tracker</a></li>
             <li><a href="https://github.com/apache/arrow">Source Code</a></li>
+          </ul>
+        </li>
+        <li class="dropdown">
+          <a href="#" class="dropdown-toggle" data-toggle="dropdown"
+             role="button" aria-haspopup="true"
+             aria-expanded="false">Community<span class="caret"></span>
+          </a>
+          <ul class="dropdown-menu">
             <li><a href="http://mail-archives.apache.org/mod_mbox/arrow-dev/">Mailing List</a></li>
+            <li><a href="https://cwiki.apache.org/confluence/display/ARROW">Developer Wiki</a></li>
             <li><a href="{{ site.baseurl }}/committers/">Committers</a></li>
             <li><a href="{{ site.baseurl }}/powered_by/">Powered By</a></li>
           </ul>
diff --git a/index.html b/index.html
index ec80075..a5fc534 100644
--- a/index.html
+++ b/index.html
@@ -13,6 +13,11 @@
     <div class="row">
         <div class="col-xs-12">
             <h4>
+              Interested in contributing? Join the <a href="http://mail-archives.apache.org/mod_mbox/arrow-dev/"><strong>mailing list</strong></a> or check out the <a href="https://cwiki.apache.org/confluence/display/ARROW"><strong>developer wiki</strong></a>.
+            </h4>
+        </div>
+        <div class="col-xs-12">
+            <h4>
                 <a href="{{ site.baseurl }}/blog/"><strong>See Latest News</strong></a>
             </h4>
         </div>
@@ -72,7 +77,7 @@
         </div>
     </div>
 </div>
-<!-- /container -->  
+<!-- /container -->
 
 </body>
 </html>