doc: link to sub-repositories from reference page

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5752049

doc/reference.html

@@ -14,6 +14,22 @@
 The documentation for the Go standard library.
 </p>
 
+<h4 id="subrepos">Sub-repositories</h4>
+
+<p>
+These packages are part of the Go Project but outside the main Go tree.
+They are developed under looser <a href="/doc/go1compat.html">compatibility
+requirements</a> than the Go core.
+Install them with "<code><a href="/cmd/go/#Download_and_install_packages_and_dependencies">go get</a></code>".
+</p>
+
+<ul>
+<li><a href="http://code.google.com/p/go/source/browse?repo=codereview"><code>code.google.com/p/go.codereview</code></a> [<a href="http://gopkgdoc.appspot.com/pkg/code.google.com/p/go.codereview">docs</a>]
+<li><a href="http://code.google.com/p/go/source/browse?repo=crypto"><code>code.google.com/p/go.crypto</code></a> [<a href="http://gopkgdoc.appspot.com/pkg/code.google.com/p/go.crypto">docs</a>]
+<li><a href="http://code.google.com/p/go/source/browse?repo=image"><code>code.google.com/p/go.image</code></a> [<a href="http://gopkgdoc.appspot.com/pkg/code.google.com/p/go.image">docs</a>]
+<li><a href="http://code.google.com/p/go/source/browse?repo=net"><code>code.google.com/p/go.net</code></a> [<a href="http://gopkgdoc.appspot.com/pkg/code.google.com/p/go.net">docs</a>]
+</ul>
+
 <h3 id="cmd"><a href="/ref/cmd">Command Documentation</a></h3>
 <p>
 The documentation for the Go tools.