Write meeting a deadline page

@rogerhutchings So we both did the same thing. I haven't done screenshots but I've written a bit more text. Might be worth committing them both together?
Does this need to be a quote?  I think it'd look better rolled into the previous paragraph:

`What milestones should all have in common, however, is a due date. An issue without a milestone has no urgency. If you need an issue closed by a certain date, make it part of a milestone.`