Added feature to print a <notice> from manifest at the end of a sync.

This feature is used to convey information on a when a branch has
ceased development or if it is an experimental branch with a few
gotchas, etc.

You add it to your manifest XML by doing something like this:
<manifest>
  <notice>
    NOTE TO DEVELOPERS:
      If you checkin code, you have to pinky-swear that it contains no bugs.
      Anyone who breaks their promise will have tomatoes thrown at them in the
      team meeting.  Be sure to bring an extra set of clothes.
  </notice>

  <remote ... />
  ...
</manifest>

Carriage returns and indentation are relevant for the text in this tag.

This feature was requested by Anush Elangovan on the ChromiumOS team.