Readd yahoo to the list of supposed sites

It has been fixed for a while.

documentation.html

@@ -65,11 +65,8 @@ For a complete list, see the <a href="supportedsites.html">Supported sites</a> p
 <li>Google Video.</li>
 <li>Google Video searches ("gvsearch" keyword).</li>
 <li>Photobucket videos.</li>
-<!--
+<li>Yahoo! screen.</li>
-Unsupported, see https://github.com/rg3/youtube-dl/issues/300
+<li>Yahoo! screen searches ("yvsearch" keyword).</li>
-<li>Yahoo! video.</li>
-<li>Yahoo! video searches ("ybsearch" keyword).</li>
--->
 <li>Dailymotion.</li>
 <li>DepositFiles.</li>
 <li>blip.tv.</li>