doc: link to iant's generics proposal from the FAQ.

Updates #15292.

Change-Id: I229f66c2a41ae0738225f2ba7a574478f5d6d620
Reviewed-on: https://go-review.googlesource.com/22163Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go_faq.html

@@ -335,7 +335,8 @@ code that does what generics would enable, if less smoothly.
 </p>
 
 <p>
-This remains an open issue.
+This remains an open issue. See <a href="https://golang.org/issue/15292">the generics proposal issue</a>
+for more information.
 </p>
 
 <h3 id="exceptions">