Add the page on the MX disk format. Mac disks are now unicorns.

doc/disk-mx.md

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+Disk: DVK MX
+============
+
+The DVK (in Russian, ДВК, Диалоговый вычислительный комплекс or Dialogue
+Computing Complex) was a late 1970s Soviet personal computer, a cut-down
+version of the professional SM EVM (СМ ЭВМ, abbreviation of Система Малых ЭВМ
+--- literally System of Mini Computers), which _itself_ was an unlicensed
+clone of the PDP-11. The MX board was an early floppy drive controller board
+for it.
+
+<div style="text-align: center">
+<a href="http://www.leningrad.su/museum/show_big.php?n=1006"><img src="dvk3m.jpg" style="max-width: 60%" alt="A Durango F85, held precariously"></a>
+</div>
+
+The MX format is interesting in that it has to be read a track at a time. The
+format contains the usual ID prologue at the beginning of the track, then
+eleven data blocks and checksums, then the epilogue, then it stops. The
+actual encoding is normal FM. There were four different disk variants, in all
+combinations of single- and double-sided and 40- and 80-tracked; but every
+track contained eleven 256-byte sectors.
+
+The format varies subtly depending on whether you're using the 'new' driver
+or the 'old' driver. FluxEngine should read both.
+
+A track is:
+
+  * 8 x 0x0000 words (FM encoded as 01010101...)
+  * 1 x 0x00F3 --- start of track
+  * 1 x 0xnnnn --- track number
+  * 11 of:
+    * 128 words (256 bytes) of data
+    * 16 bit checksum
+  * **if 'new' format:**
+    * 3 x 0x83nn --- `n = (track_number<<1) + side_number`
+  * **if 'old' format:**
+    * 3 x 0x8301
+
+The checksum is just the unsigned integer sum of all the words in the sector.
+Words are all stored little-endian.
+
+Reading discs
+-------------
+
+```
+fluxengine read mx
+```
+
+You should end up with an `mx.img` which will vary in length depending on the format. The default is double-sided 80-track. For the other formats, use:
+
+  * single-sided 40-track: `-s :s=0:t=0-79x2`
+  * double-sided 40-track: `-s :s=0-1:t=0-79x2`
+  * single-sided 40-track: `-s :s=0:t=0-79`
+  * double-sided 40-track: `-s :s=0-1:t=0-79`
+
+
+Useful references
+-----------------
+
+  - [The Soviet Digital Electronics
+    Museum](http://www.leningrad.su/museum/main.php) (source of the image
+    above)
+
+  - [a random post on the HxC2001 support
+    forum](http://torlus.com/floppy/forum/viewtopic.php?t=1384) with lots of
+    information on the format