new doc on message disposition

[Michael Goulish]

Checked in at 

    trunk/docs/messenger/message_disposition.md

This one has it all !
Windows!  Trackers!  The joy of message acceptance
and the crushing disappointment of message rejection!

<lie>
Flash animation!  Soundtrack featuring local indie rock and urban folk bands!
</lie>

Please take a look!   Comment early and often !