kexec-kdump-howto:Add introduction of parallel dumping
Add descriptions of parallel dumping and how to use it.
Signed-off-by: Zhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
Reviewed-by: Pratyush Anand <panand@redhat.com>
Acked-by: Dave Young <dyoung@redhat.com>