net/smtp: document that the smtp package is frozen

This copies the frozen wording from the log/syslog package.

Fixes #16436

Change-Id: If5d478023328925299399f228d8aaf7fb117c1b4
Reviewed-on: https://go-review.googlesource.com/25080
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/net/smtp/smtp.go

@@ -8,6 +8,11 @@
 //	AUTH      RFC 2554
 //	STARTTLS  RFC 3207
 // Additional extensions may be handled by clients.
+//
+// The smtp package is frozen and not accepting new features.
+// Some external packages provide more functionality. See:
+//
+//   https://godoc.org/?q=smtp
 package smtp
 
 import (