Add documentation for the DS990.

doc/disk-tids990.md

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+Disk: TI DS990 FD1000
+=====================
+
+The Texas Instruments DS990 was a multiuser modular computing system from 1998,
+based around the TMS-9900 processor (as used by the TI-99). It had an 8" floppy
+drive module, the FD1000, which was a 77-track, 288-byte sector FM/MFM system
+with 26 sectors per track. The encoding scheme was very similar to a simplified
+version of the IBM scheme, but of course not compatible. A double-sided disk
+would store a very satisfactory 1126kB of data; here's one at <a
+href="https://www.old-computers.com/museum/computer.asp?st=1&c=1025">old-computers.com</a>:
+
+<div style="text-align: center">
+<a href="https://www.old-computers.com/museum/computer.asp?st=1&c=1025">
+<img src="tids990.jpg" style="max-width: 60%" alt="A DS990 at old-computers.com"></a>
+</div>
+
+FluxEngine will read and write these (but only the DSDD MFM variant).
+
+Reading discs
+-------------
+
+Just do:
+
+```
+fluxengine read tids990
+```
+
+You should end up with an `tids990.img` which is 1153152 bytes long.
+
+Writing discs
+-------------
+
+Just do:
+
+```
+fluxengine write tids990 -i tids990.img
+```
+
+
+Useful references
+-----------------
+
+  - [The FD1000 Depot Maintenance
+	Manual](http://www.bitsavers.org/pdf/ti/990/disk/2261885-9701_FD1000depotVo1_Jan81.pdf)
+
+