[w3c/webauthn] 60146e: Add recommendation of minimum challenge length

  Branch: refs/heads/issue-85-challenge-recommendations
  Home:   https://github.com/w3c/webauthn
  Commit: 60146e8eb70c1014bc836314e4b4b91a196026b2
      https://github.com/w3c/webauthn/commit/60146e8eb70c1014bc836314e4b4b91a196026b2
  Author: Emil Lundberg <emil@yubico.com>
  Date:   2018-04-04 (Wed, 04 Apr 2018)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Add recommendation of minimum challenge length