PCI: PCIE AER: Document for PCIE AER software error injection

This patch adds a minimal HOWTO for PCIE AER software error injection
in Documentation/PCI/pcieaer-howto.txt.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>