Add features.md doc and link to it in the options page

The features.md doc describes all the features that the extension
provides. This change also adds a link to this doc in the options page
so it can be accessed quickly.

Fixes: #23
Change-Id: Iad1d54ba216d6dd4e7820fe2a2ed8644f86ed945
diff --git a/src/options.html b/src/options.html
index cac1de5..16b8fcf 100644
--- a/src/options.html
+++ b/src/options.html
@@ -7,6 +7,15 @@
     <link rel="stylesheet" href="options.css">
   </head>
   <body>
+    <a href="https://gerrit.avm99963.com/plugins/gitiles/infinitegforums/+/master/docs/features.md" target="_blank" class="features-link">
+      <!--
+        Material Design Icon - action/help_outline
+         - LICENSE: Apache License Version 2.0
+         - Source: https://github.com/google/material-design-icons/
+         - Author: Google LLC
+      -->
+      <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M0 0h24v24H0V0z" fill="none"/><path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm0 18c-4.41 0-8-3.59-8-8s3.59-8 8-8 8 3.59 8 8-3.59 8-8 8zm-1-4h2v2h-2zm1.61-9.96c-2.06-.3-3.88.97-4.43 2.79-.18.58.26 1.17.87 1.17h.2c.41 0 .74-.29.88-.67.32-.89 1.27-1.5 2.3-1.28.95.2 1.65 1.13 1.57 2.1-.1 1.34-1.62 1.63-2.45 2.88 0 .01-.01.01-.01.02-.01.02-.02.03-.03.05-.09.15-.18.32-.25.5-.01.03-.03.05-.04.08-.01.02-.01.04-.02.07-.12.34-.2.75-.2 1.25h2c0-.42.11-.77.28-1.07.02-.03.03-.06.05-.09.08-.14.18-.27.28-.39.01-.01.02-.03.03-.04.1-.12.21-.23.33-.34.96-.91 2.26-1.65 1.99-3.56-.24-1.74-1.61-3.21-3.35-3.47z"/></svg>
+    </a>
     <form>
       <div class="option"><input type="checkbox" id="list"> <label for="list" data-i18n="list"></label></div>
       <div class="option"><input type="checkbox" id="thread"> <label for="thread" data-i18n="thread"></label></div>