dict: document package as frozen

This package was created as an example of use of the textproto package,
and has been essentially unchanged since then. Document it as not accepting
new features.

Change-Id: I692c22836ff5a382e498ca6969c0db9ab694de49
Reviewed-on: https://go-review.googlesource.com/c/net/+/346210
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

dict/dict.go

@@ -4,6 +4,8 @@
 
 // Package dict implements the Dictionary Server Protocol
 // as defined in RFC 2229.
+//
+// The dict package is frozen and is not accepting new features.
 package dict // import "golang.org/x/net/dict"
 
 import (