Add instructions for updating launchpad

The build version number of the launchpad daily builds should match the
current development version number.
Note that we need a barnowl-1.(N+1)dev tag

Perhaps we should include the invocations of git tag and git push to do
this?
ChangeLog updates should always show up on master

This way Anders won't have to do any more commits like
1b17f506686dd0a0d0d433dec03180490d16d587 (or, at least, it won't take so
long to notice we need such commits).