overlays/README: Document that vc4-(f)kms requires >=512MB

The firmware stops vc4-kms-v3d and vc4-fkms-v3d loading if the
system has less than 512MB of RAM. It can work if gpu_mem and
CMA heap size are set appropriately, but can't be guaranteed.
Document this restriction.

Also drops Eric's name from the overlay description as it
isn't relevant or accurate anymore.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>