PCI: Document pci_ids.h addition policy.

IDs should generally only be added to pci_ids.h when they're shared
across several files in the tree.  IDs that are just used by a single
driver should be defined in the driver instead.

Perhaps documenting this is a good idea to prevent things being moved there,
as it still seems to be happening judging from the git log.

(based on discussion w/gregkh and others).

Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>