[PATCH] Document the fact that linux-arm-kernel is subscribers-only.

"Non-members are not allowed to post messages to this list.  Blame the
original poster for cross-posting to subscriber-only mailing lists.  "

Signed-off-by: Alexey Dobriyan <adobriyan@mail.ru>
Acked-by: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>