[webauthn] clarify that at least one model of attestation must be employed

equalsJeffH has just created a new issue for 
https://github.com/w3c/webauthn:

== clarify that at least one model of attestation must be employed ==
https://w3c.github.io/webauthn/#iface-attestation says..
```
4.7. Credential Attestation Statement (interface WebAuthnAttestation)

Authenticators also provide some form of attestation. The basic 
requirement 
is that the authenticator can produce, for each credential public key,
 attestation 
information that can be verified by a WebAuthn Relying Party. 
```
It should perhaps be "..MUST also provide..", and have a link to the 
{#attestation-models} section.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/115 using your GitHub account