[w3c/webauthn] f780ca: Add note on why authenticator attachment ise used ...

  Branch: refs/heads/master
  Home:   https://github.com/w3c/webauthn
  Commit: f780ca85a74f664bea4890bd1111a6e97b1099f0
      https://github.com/w3c/webauthn/commit/f780ca85a74f664bea4890bd1111a6e97b1099f0
  Author: Emil Lundberg <emil@emlun.se>
  Date:   2017-12-20 (Wed, 20 Dec 2017)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Add note on why authenticator attachment ise used only in create() (#708)