Add cheat sheet subtab under Tags tab

To provide more publicity to cheat sheet. It's an important education on how to use the site for inexperienced users.

app/views/tags/_secondary_links.html.erb

@@ -4,6 +4,7 @@
     <li><%= link_to "Listing", tags_path %></li>
     <li><%= link_to "Aliases", tag_aliases_path %></li>
     <li><%= link_to "Implications", tag_implications_path %></li>
+    <li><%= link_to "Cheatsheet", wiki_pages_path(:search => {:title => "help:cheatsheet"}) %></li>
     <li><%= link_to "Help", wiki_pages_path(:search => {:title => "help:tags"}) %></li>
     <% if @tag %>
       <li>|</li>