Add note to Consent Records guide regarding maintaining record of past consent states

[coolharsh55]

Add a note stating rationale for why consent records must be maintained for past consent states and not just the latest states, see Meeting FEB-20 for proposal from GeorgKrog

[coolharsh55]

Note is live at https://w3c.github.io/dpv/guides/consent-27560.html#consent-records-and-receipts - to be reviewed