Documentation/SubmitChecklist: add rule #1

Add header file requirements.  Compliments of Stephen Rothwell.

Stephen calls this Rule #1, so I put it there, but I didn't want to demote
any of the others in the list, so I made one of them number 2b.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>