godoc: display advert for the package dashboard on package list page

R=golang-dev, bradfitz, ality
CC=golang-dev
https://golang.org/cl/4526070

lib/godoc/package.html

@@ -91,6 +91,11 @@
 	</p>
 {.end}
 {.section Dirs}
+	<p class="detail">
+	Need more packages? The
+	<a href="http://godashboard.appspot.com/package">Package Dashboard</a>
+	provides a list of <a href="/cmd/goinstall/">goinstallable</a> packages.
+	</p>
 	{# DirList entries are numbers and strings - no need for FSet}
 	<h2 id="Subdirectories">Subdirectories</h2>
 	<p>