Replace the content of the qnx4 README file with the canonical place for
such information.

Add the credits of the qnx4 contribution to CREDITS. As there is already a
QNX4 FILESYSTEM section in MAINTAINERS, it is clear who to contact and send
patches to.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-By: Anders Larsen <al@alarsen.net>
Link: https://lore.kernel.org/r/20230220170210.15677-3-lukas.bulwahn@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>