[w3c/webauthn] b9af92: Add recommendation of challenge length (#858)

  Branch: refs/heads/master
  Home:   https://github.com/w3c/webauthn
  Commit: b9af923897efecaf2f85558748ee3fe3385fddd2
      https://github.com/w3c/webauthn/commit/b9af923897efecaf2f85558748ee3fe3385fddd2
  Author: Emil Lundberg <emil@emlun.se>
  Date:   2018-04-16 (Mon, 16 Apr 2018)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Add recommendation of challenge length (#858)

LGTM, merging per 11-Apr-2018 concall.

* Add recommendation of minimum challenge length

* Add link to security consideration from makeCredential challenge parameter