doc: suggest security@golang.org for reporting security issues

Fixes #11402

Change-Id: I195575a54e30ad6d6b1d0996fde34c0163bbd2c2
Reviewed-on: https://go-review.googlesource.com/11530Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTING.md

@@ -18,6 +18,8 @@ When filing an issue, make sure to answer these five questions:
 General questions should go to the [golang-nuts mailing list](https://groups.google.com/group/golang-nuts) instead of the issue tracker.
 The gophers there will answer or ask you to file an issue if you've tripped over a bug.
 
+Sensitive security-related issues should be reported to [security@golang.org](mailto:security@golang.org).
+
 ## Contributing code
 
 Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html)

doc/contrib.html

@@ -91,6 +91,10 @@ a new one.)
 We pride ourselves on being meticulous; no issue is too small.
 </p>
 
+<p>
+Sensitive security-related issues should be reported to <a href="mailto:security@golang.org">security@golang.org</a>.
+</p>
+
 <h3><a href="/doc/contribute.html">Contributing code</a></h3>
 
 <p>