doc: add Go Security Policy document

Bring in the text from the proposal (with minor edits):
https://github.com/golang/proposal/blob/master/design/11502-securitypolicy.md

Fixes #11502

Change-Id: I92a987be66a0df60c1fad6c6c79f89bd8e9c12a8
Reviewed-on: https://go-review.googlesource.com/13955
Reviewed-by: Jason Buberel <jbuberel@google.com>
diff --git a/doc/contrib.html b/doc/contrib.html
index 5667491..b3ca155 100644
--- a/doc/contrib.html
+++ b/doc/contrib.html
@@ -90,7 +90,9 @@
 </p>
 
 <p>
-Sensitive security-related issues should be reported to <a href="mailto:security@golang.org">security@golang.org</a>.
+Security-related issues should be reported to
+<a href="mailto:security@golang.org">security@golang.org</a>.
+See the <a href="/security">security policy</a> for more details.
 </p>
 
 <h3><a href="/doc/contribute.html">Contributing code</a></h3>