Add BK documentation.

README.md

@@ -103,6 +103,7 @@ people who've had it work).
 | [Brother 120kB](doc/disk-brother.md)      |  🦄   |   🦄   |                                     |
 | [Brother 240kB](doc/disk-brother.md)      |  🦄   |   🦄   |                                     |
 | [Brother FB-100](doc/disk-fb100.md)       |  🦖   |        | Tandy Model 100, Husky Hunter, knitting machines |
+| [Elektronika BK](doc/disk-bd.md)          |  🦄   |   🦄   | Soviet Union PDP-11 clone           |
 | [Macintosh 400kB/800kB](doc/disk-macintosh.md)  |  🦄   |   🦄   |                                     |
 | [NEC PC-98](doc/disk-ibm.md)              |  🦄   |   🦄   | trimode drive not required          |
 | [Sharp X68000](doc/disk-ibm.md)           |  🦄   |   🦄   |                                     |

doc/disk-bk.md

@@ -0,0 +1,37 @@
+Disk: Elektronica BK
+====================
+
+The BK (an abbreviation for бытовой компьютер --- 'home computer' in Russian)
+is a Soviet era personal computer from Elektronika based on a PDP-11
+single-chip processor. It was the _only_ official, government approved home
+computer in mass production at the time.
+
+It got a floppy interface in 1989 when the 128kB BK-0011 was released. This
+used a relatively normal double-sided IBM scheme format with 80 sectors and ten
+sectors per track, resulting in 800kB disks. The format is, in fact, identical
+to the Atari ST 800kB format. Either 5.25" or 3.5" drives were used depending
+on what was available at the time, with the same format on both.
+
+Reading disks
+-------------
+
+Just do:
+
+```
+fluxengine read bk800 -o bk800.img
+```
+
+You should end up with an `bk800.img` containing all the sectors concatenated
+one after the other in CHS order. This will work on both 5.25" and 3.5" drives.
+
+Writing disks
+-------------
+
+Just do:
+
+```
+fluxengine write bk800 -i bk800.img
+```
+
+This will write the disk image to either a 5.25" or 3.5" drive.
+