doc: add more gophers

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5753047
diff --git a/doc/contrib.html b/doc/contrib.html
index 76cee1f..7a96b8d 100644
--- a/doc/contrib.html
+++ b/doc/contrib.html
@@ -3,6 +3,8 @@
 	"Path": "/project/"
 }-->
 
+<img class="gopher" src="/doc/gopher/project.png" />
+
 <div id="manual-nav"></div>
 
 <p>