rpi4: Add initial documentation file

As the Raspberry Pi4 port is now in a usable state, add the build
instructions together with some background information to the
documentation directory.
The port differs quite a bit from the Raspberry Pi 3, so we use a
separate file for that.

Change-Id: I7d9f5967fdf3ec3bfe97d78141f59cbcf03388d4
Signed-off-by: Andre Przywara <andre.przywara@arm.com>