Adding RELEASEDIR/iso to release(7)

The new iso-image code recently committed by Manuel Bouyer builds small
CD-ROM images in RELEASEDIR/MACHINE/install/cdrom/*, and large CD-ROM
images in RELEASEDIR/iso/*.  The small images tend to be for exactly
one platform, and contain a kernel, sysinst, some crunchgenned tools,
but not *.tgz sets.  The large images contain *.tgz sets, and could in
principle be multi-platform.

releases(7) doesn't talk about RELEASEDIR/iso at all.  I'd like to
update it, but first I'd like to understand the difference between the
two locations.  Given a CD-ROM image, how would you determine which
directory it belongs in, and how would you choose the file name?

--apb (Alan Barrett)