Add support for reading ADFS disks --- a slight variation to normal MFM IBM.

Made the IBM reader way more robust in terms of retrying bad reads, as my
sample disks were pretty dodgy.

doc/acorn-adfs.md

@@ -0,0 +1,35 @@
+Acorn ADFS disks
+================
+
+Acorn ADFS disks are pretty standard MFM encoded IBM scheme disks, although
+with different sector sizes and with the 0-based sector identifiers rather
+than 1-based sector identifiers. There's nothing particularly special here.
+
+There are various different kinds, which should all work out of the box.
+Tested ones are:
+
+  - ADFS L: 80 track, 16 sector, 2 sides, 256 bytes per sector == 640kB.
+  - ADFE D/E: 80 track, 5 sector, 2 sides, 1024 bytes per sector == 800kB.
+
+I expect the others to work, but haven't tried them; [get in
+touch](https://github.com/davidgiven/fluxengine/issues/new) if you have any
+news. For ADFS S (single sided 40 track) you'll want `-s :s=0:t=0-79x2`. For
+ADFS M (single sided 80 track) you'll want `-s :s=0`.
+
+Be aware that Acorn logical block numbering goes all the way up side 0 and
+then all the way up side 1. However, FluxEngine uses traditional disk images
+with alternating sides, with the blocks from track 0 side 0 then track 0 side
+1 then track 1 side 0 etc. Most Acorn emulators will use both formats, but
+they might require nudging as the side order can't be reliably autodetected.
+
+Reading discs
+-------------
+
+Just do:
+
+```
+.obj/fe-readibm --sector-id-base=0
+```
+
+You should end up with an `ibm.img` of the appropriate size for your disk
+format.