doc: mention security from contribution guidelines

Fixes #15413

Change-Id: I837a391276eed565cf66d3715ec68b7b959ce143
Reviewed-on: https://go-review.googlesource.com/22390Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/contribute.html

@@ -16,6 +16,10 @@ have <a href="code.html">written and tested your code</a>.
 see <a href="gccgo_contribute.html">Contributing to gccgo</a>.)
 </p>
 
+<p>
+Sensitive security-related issues should be reported to <a href="mailto:security@golang.org">security@golang.org</a>.
+</p>
+
 <h2 id="Design">Discuss your design</h2>
 
 <p>