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

Documentation/SubmitChecklist

@@ -9,10 +9,14 @@ Documentation/SubmittingPatches and elsewhere regarding submitting Linux
 kernel patches.
 
 
-1: Builds cleanly with applicable or modified CONFIG options =y, =m, and
+1: If you use a facility then #include the file that defines/declares
+   that facility.  Don't depend on other header files pulling in ones
+   that you use.
+
+2: Builds cleanly with applicable or modified CONFIG options =y, =m, and
    =n.  No gcc warnings/errors, no linker warnings/errors.
 
-2: Passes allnoconfig, allmodconfig
+2b: Passes allnoconfig, allmodconfig
 
 3: Builds on multiple CPU architectures by using local cross-compile tools
    or some other build farm.