doc: mention security from contribution guidelines

Fixes #15413

Change-Id: I837a391276eed565cf66d3715ec68b7b959ce143
Reviewed-on: https://go-review.googlesource.com/22390
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/doc/contribute.html b/doc/contribute.html
index 59b3d20..1cd6f37 100644
--- a/doc/contribute.html
+++ b/doc/contribute.html
@@ -16,6 +16,10 @@
 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>