Polina Goltsman issue 14

"sec. 3.2.1
==========
      clients that choose this option need to process Update messages

Update messages are not introduced yet, so a forward reference to
section 6.4.2.3 would be helpful"

- As suggested.
Merge pull request #221 from petithug/polina-goltsman-issue-14

Polina Goltsman issue 14