proposal x68k/stand/README

I'd like to add such documentation as arch/x68k/stand/README.
But I don't know details so please review.

As a background, arch/x68k/stand/ has many directories and
there are three stage bootloaders mixed.  Every time I enter
this directory every few years, I don't remember which is which.

Some port already have its arch/xx/stand/README.  Therefore
I think it's nice that x68k has stand/README too.

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$NetBSD$

Primary bootloaders:
	boot_ufs/
		contains xxboot_ufs.
		It is placed in FFS's boot area and loads secondary bootloader
		from its filesystem (FFSv1/v2).

	boot_ustar/
		is placed in floppy's sector#0 and loads secondary bootloader
		from subsequent USTARFS.

	xxboot/
		contains following variants.
		* cdboot_cd9660
			is placed in CD and loads secondary bootloader
			from cd9660 filesystem.
		* xxboot_ffsv1
		* xxboot_ffsv2
		* xxboot_lfsv1
		* xxboot_lfsv2
			is placed in each specified filesystem's boot area,
			and loads secondary bootloader from its filesystem.

Secondary bootloaders:
	boot/
		boot is placed to / (root directory) and loads the kernel.
	netboot/
		netboot is the same as /boot except default boot device is
		net.

Other bootloaders:
	mboot/
		mboot is 0-th bootloader.
		It is placed to sector#0 (as 1024byte/sector) of hard disk
		as an alternative to genuine boot selector.  And loads
		active partition's +0 sector (sector size depends media)
		which should have primary bootloader.

	loadbsd/
		loadbsd.x is a Human68k executable and loads the kernel from
		Human68k filesystem.
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Thanks,
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Tetsuya Isaki <[email protected] / [email protected]>