How to add a Received-Spf header?

[Rob Arista]

Hey.

I'm doing SPF checking in Spamassassin.  I read the logs ans see the
tests done.  And I get scores in the X-Spam-Report headers.

I can't see how to get SA to add a 'Received-Spf' like described here,
to use it further downstream:

http://www.openspf.org/SPF_Received_Header

The following key-value pairs are designed for later machine parsing.
SPF clients SHOULD give enough information so that the SPF results can
be verified. That is, at least "client-ip", "helo", and, if the "MAIL
FROM" identity was checked, "envelope-from".
...
Here is an example:

    Received-SPF: Pass (example.org: domain of sc...@kitterman.com
        designates 72.81.252.18 as permitted sender)
        receiver=example.org; client-ip=72.81.252.18;
        envelope-from=<sc...@kitterman.com>; helo=mailout00.controlledmail.com;

How do I instruct SA to add that specific header & info?

--Bob