fault-injection: mention failcmd.sh tool in document

fault-injection: mention failcmd.sh tool in document

fault-injection-add-tool-to-run-command-with-failslab-or-fail_page_alloc.patch
in -mm tree adds tools/testing/fault-injection/failcmd.sh to make it
easier to inject slab/page allocation failures by fault injection.

This adds the introduction to
Documentation/fault-injection/fault-injection.txt.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>