[w3c/webauthn] 8c2bfd: add explanation: where does signature come from

  Branch: refs/heads/sign-counter-alg-507
  Home:   https://github.com/w3c/webauthn
  Commit: 8c2bfd70057b3e958532a89b10f6cf48cf462fa6
      https://github.com/w3c/webauthn/commit/8c2bfd70057b3e958532a89b10f6cf48cf462fa6
  Author: rlin1 <rolf@noknok.com>
  Date:   2017-08-30 (Wed, 30 Aug 2017)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  add explanation: where does signature come from