Documentation/SubmittingPatches: describe the Fixes: tag

Update the SubmittingPatches process to include howto about the new
'Fixes:' tag to be used when a patch fixes an issue in a previous commit
(found by git-bisect for example).

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>