[webauthn] assertionChallenge recommendations

apowers313 has just created a new issue for 
https://github.com/w3c/webauthn:

== assertionChallenge recommendations ==
Should we include some recommendations on the size / format of the 
assertion challenge? I had arbitrarily selected 256 random bytes in my
 server implementation, but that seems like a burden for NFC / BLE 
authenticators -- something that I hadn't thought about until now.

I just figure that a hint or two might prevent people who are 
unfamiliar with the system as a whole from creating unintentional 
problems.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/85 using your GitHub account