Document our TSA/signed timestamp policy

[woodruffw]

We should do this before making a public release that includes TSA/timestamp support:

• Explicitly document our signing policy: when signing, sigstore-python will attempt to contact every TSA in the trust root, obtain a signed timestamp, and will embed those signed timestamps in the bundle
• Explicitly document our verification policy: when verifying, sigstore-python will attempt to verify each timestamp response, but only requires a threshold of 1-of-N. Moreover, the integration time from the tlog itself is still treated as a source of signed time.

I think the only open question is where in the code/docs these notes should live 🙂

CC @jku