[w3c/webauthn] 175813: Add note on why authenticator attachment ise used ...

  Branch: refs/heads/issue-420-attachment
  Home:   https://github.com/w3c/webauthn
  Commit: 175813448d8bda75e3d7e5fa97e53ea1973af9ed
      https://github.com/w3c/webauthn/commit/175813448d8bda75e3d7e5fa97e53ea1973af9ed
  Author: Emil Lundberg <emil@yubico.com>
  Date:   2017-12-06 (Wed, 06 Dec 2017)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Add note on why authenticator attachment ise used only in create()