At Last! Xamarin Forms 2.2.0 includes Margins support!
Here are the docs with a great visual.

UPD Special for @AUSTX_RJL
Margin value is Thickness, just like in any other XAML frameworks.
You can set Thickness in XAML by setting one, two or four values separated by comma or whitespace:
"1 2 3 4" is same as "1, 2, 3, 4" and sets:
- 1 for
Left - 2 for
Top - 3 for
Right - 4 for
Bottom
Fields of Thickness
"1 2" sets:
- 1 for
LeftandRightfields - 2 for
TopandBottomfields
"1" sets 1 for all fields of Thickness