doc: add link to codewalks.

Fixes #1281.

R=adg
CC=golang-dev
https://golang.org/cl/3150041

doc/docs.html

diff --git a/doc/docs.html b/doc/docs.html
index 6643b20..9ffa022 100644
--- a/doc/docs.html
+++ b/doc/docs.html
@@ -47,6 +47,11 @@
 and closures.
 </p>
 
+<h3 id="codewalks"><a href="codewalk/">Codewalks</a></h3>
+<p>
+Guided tours of Go programs. 
+</p>
+
 <h3 id="go_for_cpp_programmers"><a href="go_for_cpp_programmers.html">Go for C++ Programmers</a></h3>
 <p>
 An introduction to Go for C++ programmers.