pkcs12: note that this package is frozen and point to an alternative

Change-Id: Ia4d87b816f176194cd9e37efb42e273d678a84b7
Reviewed-on: https://go-review.googlesource.com/c/160259
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

pkcs12/pkcs12.go

@@ -7,6 +7,9 @@
 // This implementation is distilled from https://tools.ietf.org/html/rfc7292
 // and referenced documents. It is intended for decoding P12/PFX-stored
 // certificates and keys for use with the crypto/tls package.
+//
+// This package is frozen. If it's missing functionality you need, consider
+// an alternative like software.sslmate.com/src/go-pkcs12.
 package pkcs12
 
 import (