Added Z2 System Use Entry Format to zisofs2 specs

doc/zisofs2_format.txt

@@ -12,7 +12,7 @@ The zisofs2 format was invented by Valentín KIVACHUK BURDÁ and
 Thomas SCHMITT (as extension of zisofs by H. Peter Anvin). It compresses
 data file content, marks it by a header and provides a pointer array for 
 coarse random access. Within a RRIP enhanced ISO 9660 image the format 
-is additionally marked by a System Use entry with signature "ZF".
+is additionally marked by a System Use entry with signature "ZF" or "Z2".
 
 The uncompressed size of a single zisofs2 compressed file is restricted
 to 2^64 - 1 bytes. Larger files shall not be compressed. 
@@ -146,6 +146,25 @@ Example (block size 128 kiB, uncompressed file size = 40 TB):
    0x00, 0x80, 0xCA, 0x39, 0x61, 0x24, 0x00, 0x00 }
 
 
+                         Z2 System Use Entry Format
+
+Linux kernels which are configured by CONFIG_ZISOFS to recognize zisofs
+but are not aware of zisofs2 will complain about ZF entries which announce
+algorithms other than "pz". The system log will show for each zisofs2
+compressed file at first stat(2) or open(2) a line like this:
+
+  isofs: Unknown ZF compression algorithm: PZ
+
+To avoid these complaints, it is possible to use 
+
+  [1] "BP 1 to BP 2 - Signature Word" shall be (5A)(32) ("Z2").
+
+instead of "ZF" with the System Use Entry Format specified above.
+Everything else shall be like in ZF format version 2, including the version
+number itself:
+
+  [3] "BP 4 - System Use Entry Version" shall be 2 as in ISO 9660:7.1.1.
+
 
                                  **LEGACY**
 
@@ -225,6 +244,10 @@ SUSP 1.12
 RRIP 1.12
   ftp://ftp.ymi.com/pub/rockridge/rrip112.ps
 
+zisofs version 1
+  libisofs-*/doc/zisofs_format.txt
+  https://dev.lovelyhq.com/libburnia/libisofs/raw/branch/master/doc/zisofs_format.txt
+
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