doc: put Release History link on 'Documentation' page

Also put Weekly Snapshot History link on 'Contributing' page

Fixes #1885.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4559049

doc/contrib.html

@@ -49,7 +49,10 @@ systems and architectures.</p>
 <p>Features and ideas being developed or discussed by the Go team.</p>
 
 <h3 id="release"><a href="devel/release.html">Release History</a></h3>
-<p>A summary of the changes between tagged releases of Go.</p>
+<p>A summary of the changes between Go releases.</p>
+
+<h3 id="release"><a href="devel/weekly.html">Weekly Snapshot History</a></h3>
+<p>A summary of the changes between weekly snapshots of Go.</p>
 
 <h3 id="golang-dev"><a href="http://groups.google.com/group/golang-dev">Developer Mailing List</a></h3>
 <p>The <a href="http://groups.google.com/group/golang-dev">golang-dev</a>

doc/docs.html

@@ -113,6 +113,9 @@ The built-in documentation for the Go tools.
 The official Go Language specification. 
 </p>
 
+<h3 id="release"><a href="devel/release.html">Release History</a></h3>
+<p>A summary of the changes between Go releases.</p>
+
 <h3 id="go_mem"><a href="go_mem.html">The Go Memory Model</a></h3>
 <p>
 A document that specifies the conditions under which reads of a variable in