Add a prominent Develop link in the documentation (Closes #155)

index.html

@@ -19,6 +19,7 @@
 <table border="0" id="rgb"><tr>
 	<tr><td><a class="button" id="r" href="documentation.html">Documentation</a></td></tr>
 	<tr><td><a class="button" id="g" href="download.html">Download it</a></td></tr>
+	<tr><td><a class="button" id="y" href="https://github.com/rg3/youtube-dl/">Develop</a></td></tr>
 	<tr><td><a class="button" id="b" href="about.html">About</a></td></tr>
 </tr></table>
 

style.css

@@ -128,3 +128,7 @@ tt {
 	background-color: #444488;
 	border: 2px solid #000088;
 }
+#y {
+	background-color: #888844;
+	border: 2px solid #888800;
+}