Add information about "developer mode" to "physical device" section

[RiRa12621]

Interacting with iPhones physicall is a little tricky and it'd be great to be explicit here and name all the steps, including that you need to set your phone to "developer mode --> enabled"

docs.fyne.io/started/mobile.md

Line 43 in ed831ec

the `myapp.app` icon onto your app list.

[andydotxyz]

A good point. That requirement was added since the document was written - it is now buried so far that each time I set up a new device I read the Apple docs!