features/reset_to_bl31: add documentation for reset to bl31

Add documentation that provides an overview of the Reset to BL31
feature and the steps to build and boot a platform with this feature
enabled.

Signed-off-by: Rakshit Goyal <rakshit.goyal@arm.com>
Change-Id: I11d610b1e894f9eb13739e4d43d085ee852614e8