sheriff-o-matic: add a help link

Tools/GardeningServer/ui/ct-unexpected-failures.html

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <link rel="import" href="../bower_components/paper-button/paper-button.html">
 <link rel="import" href="../model/tree-status.html">
 <link rel="import" href="ct-failure-stream.html">
 <link rel="import" href="ct-last-updated.html">
@@ skipping 18 matching lines @@
 
       .toolbar {
         display: flex;
         justify-content: space-between;
         align-items: baseline;
         padding: 0 5px;
       }
     </style>
     <div class="toolbar">
       <a href="https://code.google.com/p/chromium/wiki/UsefulURLs">Useful URLs</a>
+      <a href="http://www.chromium.org/developers/tree-sheriffs/sheriff-details-chromium">How to Sheriff</a>
     </div>
     <ct-tree-status status="{{ treeStatuses['chromium'] }}" state="{{ treeStatuses['chromium'].status }}"></ct-tree-status>
     <template if="{{ tree == 'blink' }}">
       <ct-tree-status status="{{ treeStatuses['blink'] }}" state="{{ treeStatuses['blink'].status }}"></ct-tree-status>
     </template>
     <template if="{{ failures && failures.failures && (!failures.failures[tree] || !failures.failures[tree].length) }}">
       <ct-party-time></ct-party-time>
     </template>
     <ct-failure-stream title="Tree closers" category="treecloser" groups="{{ failures && failures.failures[tree] }}" commitLog="{{ commitLog }}"></ct-failure-stream>
     <ct-failure-stream title="Probably-hung bots" category="builder" groups="{{ failures && failures.failures[tree] }}" commitLog="{{ commitLog }}"></ct-failure-stream>
@@ skipping 21 matching lines @@
       update: function() {
         // FIXME: These shouldn't update if there's already an update in progress.
         Object.keys(this.treeStatuses, function(tree, status) {
           status.update();
         });
       },
     });
   })();
   </script>
 </polymer-element>