Add a link to upstream documentation

Closes: libevent/libevent#782

Author: azat

index.html

@@ -124,7 +124,11 @@ <h3>Sponsors</h3>
 <h3>Documentation</h3>
 Book: <a href="http://www.wangafu.net/~nickm/libevent-book/">Programming with Libevent</a>.<br>
 Reference:
-  <a href="http://www.wangafu.net/~nickm/libevent-1.4/doxygen/html">1.4.x-stable</a>&ensp;<a href="http://www.wangafu.net/~nickm/libevent-2.0/doxygen/html">2.0.x-stable</a>&ensp;<a href="http://www.wangafu.net/~nickm/libevent-2.1/doxygen/html">2.1.x-stable</a>.<br>
+<a href="http://www.wangafu.net/~nickm/libevent-1.4/doxygen/html">1.4.x-stable</a>
+<a href="http://www.wangafu.net/~nickm/libevent-2.0/doxygen/html">2.0.x-stable</a>
+<a href="http://www.wangafu.net/~nickm/libevent-2.1/doxygen/html">2.1.x-stable</a>
+<a href="/doc">master</a>
+<br>
 What's new in:
 <a href="https://raw.githubusercontent.com/libevent/libevent/patches-2.0/whatsnew-2.0.txt">2.0.x-stable</a>&ensp;
 <a href="https://raw.githubusercontent.com/libevent/libevent/patches-2.1/whatsnew-2.1.txt">2.1.x-alpha</a>.