doc: make golang-nuts discussion list more prominent

The discussion list was buried beneath the developer mailing list.
This change puts the discussion list first and gives it a more
prominent heading.

Change-Id: I8dcb4af98e454ae3a0140f9758a5656909126983
Reviewed-on: https://go-review.googlesource.com/134136Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contrib.html

@@ -59,6 +59,15 @@ Go 1 matures.
 <h3 id="source"><a href="https://golang.org/change">Source Code</a></h3>
 <p>Check out the Go source code.</p>
 
+<h3 id="discuss"><a href="//groups.google.com/group/golang-nuts">Discussion Mailing List</a></h3>
+<p>
+A mailing list for general discussion of Go programming.
+</p>
+<p>
+Questions about using Go or announcements relevant to other Go users should be sent to
+<a href="//groups.google.com/group/golang-nuts">golang-nuts</a>.
+</p>
+
 <h3 id="golang-dev"><a href="https://groups.google.com/group/golang-dev">Developer</a> and
 <a href="https://groups.google.com/group/golang-codereviews">Code Review Mailing List</a></h3>
 <p>The <a href="https://groups.google.com/group/golang-dev">golang-dev</a>
@@ -66,9 +75,6 @@ mailing list is for discussing code changes to the Go project.
 The <a href="https://groups.google.com/group/golang-codereviews">golang-codereviews</a>
 mailing list is for actual reviewing of the code changes (CLs).</p>
 
-<p>For general discussion of Go programming, see <a
-href="https://groups.google.com/group/golang-nuts">golang-nuts</a>.</p>
-
 <h3 id="golang-checkins"><a href="https://groups.google.com/group/golang-checkins">Checkins Mailing List</a></h3>
 <p>A mailing list that receives a message summarizing each checkin to the Go repository.</p>
 
@@ -116,7 +122,7 @@ To get started, read these <a href="/doc/contribute.html">contribution
 guidelines</a> for information on design, testing, and our code review process.
 </p>
 <p>
-Check <a href="//golang.org/issue">the tracker</a> for 
+Check <a href="//golang.org/issue">the tracker</a> for
 open issues that interest you. Those labeled
 <a href="https://github.com/golang/go/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22">help wanted</a>
 are particularly in need of outside help.