Add note about edit placements feature

[rtpHarry]

From gitter chat room:

Jean Jordaan @jean 06:16 if we're adding fields to a content type via the admin interface, can we re-order them?

Matthew Harris @rtpHarry 08:57 @jean Admin > Content Definition > (your type) > Edit > then in the top right there is an Edit Placement button which lets you re-organise the admin page.

Jean Jordaan @jean 09:04 @rtpHarry thanks! This might be good to mention at http://docs.orchardproject.net/en/latest/Documentation/Creating-custom-content-types/ ... I'll see if I can find the doc source to update.