Add some F85 documentation.

README.md

@@ -96,6 +96,7 @@ at least, check the CRC so what data's there is probably good.
 | Format                                   | Read? | Write? | Notes |
 |:-----------------------------------------|:-----:|:------:|-------|
 | [AES Superplus / No Problem](doc/disk-aeslanier.md) |  🦖   | | hard sectors! |
+| [Durango F85](doc/disk-durangof85.md)    |  🦖   |        | 8-inch _and_ hard sectors |
 | [Victor 9000](doc/disk-victor9k.md)      |  🦖   |        | 8-inch        |
 | [Zilog MCZ](doc/disk-zilogmcz.md)        |  🦖   |        | 8-inch _and_ hard sectors |
 {: .datatable }

doc/disk-durangof85.md

@@ -0,0 +1,45 @@
+Disk: Durango F85
+=================
+
+The Durango F85 was an early office computer based around an 8085 processor,
+sold in 1977. It ran its own multitasking operating system call DX-85M, as
+well as CP/M. It had an interesting electric-typewriter form factor, with a
+little monitor sitting on the side of it --- in operation you were facing the
+14" printer.
+
+It was touted as being portable. Which it was, if you were strong; the story
+is that they had to do an extensive search to find someone capable of lifting
+it for the following photo...
+
+<div style="text-align: center">
+<img src="durangof85.jpg" style="max-width: 60%" alt="A Durango F85, held precariously">
+</div>
+
+...and even then, only for a few seconds.
+
+It used 5.25 soft-sectored disks storing an impressive-for-those-days
+480kBish on a side, using a proprietary 4-in-5 GCR encoding. They used 77
+tracks, 12 sectors and 512 bytes per sector. Later models used double-sided
+disks; I don't have access to an image of one so don't know how they work
+(there's a suspicious looking spare byte in the sector header which could
+store the side). As always, if you have one, please [get in
+touch](https://github.com/davidgiven/fluxengine/issues/new).
+
+Reading discs
+-------------
+
+Just do:
+
+```
+.obj/fe-readf85
+```
+
+You should end up with an `f85.img` which is 472064 bytes long.
+
+Useful references
+-----------------
+
+There's amazingly little information about these things.
+
+  * [Chuck Guzis' F85 page](http://www.sydex.com/durango/durango.html) with lots of pictures
+