_content/doc/index: add link to contribution guide

Fixes golang/go#50849

Change-Id: Ifd74619ce376d9d5ebff9698ce55611dab20421a
Reviewed-on: https://go-review.googlesource.com/c/website/+/381295
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Trust: Ian Lance Taylor <iant@google.com>
diff --git a/_content/doc/index.html b/_content/doc/index.html
index bd872b6..6129ca5 100644
--- a/_content/doc/index.html
+++ b/_content/doc/index.html
@@ -257,6 +257,9 @@
 same variable in a different goroutine.
 </p>
 
+<h3 id="contributing"><a href="/doc/contribute">Contribution Guide</a></h3>
+<p>Contributing to Go.</p>
+
 <h3 id="release"><a href="/doc/devel/release.html">Release History</a></h3>
 <p>A summary of the changes between Go releases.</p>