Add section/sub-section on attestation

[adammontville]

During the September virtual interim, we had a discussion about the reliability of collected data. For example, we may believe that a specific browser is installed on a given device by observing network information (i.e. header information), and we may also observe that the actual browser installed is different by querying the device directly. The conversation turned to the topic of attestation, and we thought of RATS.

We concluded that the SACM architecture probably does not want to touch on attestation directly, but would prefer to acknowledge the work going on in other working groups, such as RATS.

Kathleen offered to write up this text.