landlock: Explain how to support Landlock

Let's help users by documenting how to enable and check for Landlock in
the kernel and the running system.  The userspace-api section may not be
the best place for this but it still makes sense to put all the user
documentation at the same place.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220513112743.156414-1-mic@digikod.net


Reviewed-by: Paul Moore <paul@paul-moore.com>