aco: Add README which explains about what ACO is and how it works.

aco: Add README which explains about what ACO is and how it works.

This is based on an early summary written by Daniel, but updated to
reflect the current state of ACO and reworded to better fit the format.
Also added is a table which details what SW stages correspond to what
HW stages on each HW generation.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Tony Wasserka <tony.wasserka@gmx.de>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4164>