kdump: add /sys/kernel/vmcoreinfo ABI documentation

/sys/kernel/vmcoreinfo was introduced long back but there is no ABI
documentation.  This patch adds the documentation.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
Cc: Dan Aloni <da-x@monatomic.org>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-kernel-vmcoreinfo

+What:		/sys/kernel/vmcoreinfo
+Date:		October 2007
+KernelVersion:	2.6.24
+Contact:	Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
+		Kexec Mailing List <kexec@lists.infradead.org>
+		Vivek Goyal <vgoyal@redhat.com>
+Description
+		Shows physical address and size of vmcoreinfo ELF note.
+		First value contains physical address of note in hex and
+		second value contains the size of note in hex. This ELF
+		note info is parsed by second kernel and exported to user
+		space as part of ELF note in /proc/vmcore file. This note
+		contains various information like struct size, symbol
+		values, page size etc.