[dovecot/core] 5af1ff: Update README regarding supported RFCs.

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 5af1ff01b69caa54039d14148055cdb20140f0a4
      https://github.com/dovecot/core/commit/5af1ff01b69caa54039d14148055cdb20140f0a4
  Author: Stephan Bosch <stephan.bosch at dovecot.fi>
  Date:   2017-12-12 (Tue, 12 Dec 2017)

  Changed paths:
    M README.md

  Log Message:
  -----------
  Update README regarding supported RFCs.

Some were missing and new ones are added by the recent lib-smtp and submission service changes.