Created an instructions dialog, on large devices.

Alerts the user that they need to drag the centerpoint around the map to choose
the spot. Is intended to work around a common problem where users misplace
points because they don't understand the point will be placed at the centerpoint
(or they don't even see it!)

The behaviour can be a little strange, due to the different ways that dialogs can open etc. Also the dialog disappears if they use location services to get their location (although having the point at their location is often what they want, so this is not so bad). But, even given the above we feel it's better with the dialog than without.

Very open to suggestions on how to improve the behaviour etc.