Add links to addon docs in mark-selection demo

Author: marijnh

demo/markselection.html

@@ -47,6 +47,6 @@ <h2>Selection Marking Demo</h2>
 editor.markText({line: 6, ch: 26}, {line: 6, ch: 42}, {className: "styled-background"});
 </script>
 
-    <p>Simple addon to easily mark (and style) selected text.</p>
+    <p>Simple addon to easily mark (and style) selected text. <a href="../doc/manual.html#addon_mark-selection">Docs</a>.</p>
 
   </article>