Document PZI0 bank format

[OPNA2608]

Current documentation only refers to PZI1:

libopenpmd/docs/pzi.txt

Lines 5 to 6 in f8720d0

	0x0000 Identifier (4b)
	"PZI1"

The bank piano.pzi included in the freeware version PMD48O_F.LZH, subarchive ppz8_sam.lzh contains a PPZ bank with the magic number PZI0. Haven't checked yet how those formats differ, but e.g. PZIUTYMX refuses to handle PZI0 banks.

[OPNA2608]

I've only seen 2 examples of PZI0 banks and both seemed to be identical in structure to PZI1 banks, safe for the magic number.

It's possible that there are some never-used (/seen) settings hiding in those 28 null bytes? But the likely explanation might be that PZI0 was a WIP version of PZI1 which was so feature complete around the time people started using it that the author just bumped the internal identifier up to PZI1. At least, that what it looks like to me.