atp: fix failing test_readme

Test failed because there was no reference to maintainers in the
meta-atp README.

Following the common structure of other layers in the meta-arm
repository, the README in meta-atp now refers to the top-level README,
and a documentation directory contains the guidance that was present in
the original meta-atp README.

Signed-off-by: Adrián Herrera Arcila <adrian.herrera@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>