doc: link to release history from /doc/

Fixes #8168.

TBR=bradfitz
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/107950043

doc/docs.html

@@ -97,6 +97,9 @@ one goroutine can be guaranteed to observe values produced by writes to the
 same variable in a different goroutine.
 </p>
 
+<h3 id="release"><a href="/doc/devel/release.html">Release History</a></h3>
+<p>A summary of the changes between Go releases.</p>
+
 
 <h2 id="articles">Articles</h2>