Thomson FD: doc + change default

I believe 1s320 is the most commonly used format in the community.

src/greaseweazle/image/fd.py

@@ -4,12 +4,19 @@
 #
 # This is free and unencumbered software released into the public domain.
 # See the file COPYING for more details, or visit <http://unlicense.org>.
+#
+# FD is a widely used format in the Thomson MO/TO community, directly
+# supported by emulators such as DCMOTO.
+#
+# The main difference between IMG and FD is that FD includes the two sides of
+# a double-sided disk sequentially, rather than interleaved per-cylinder.
+# That is, all tracks of side 0 are stored before all tracks of side 1.
 
 from greaseweazle.image.img import IMG
 
 class FD(IMG):
-    default_format = 'thomson.1s80'
-    sequential = True
+    default_format = 'thomson.1s320'    # Most commonly used format
+    sequential = True                   # whole side 0 /then/ whole side 1
 
 # Local variables:
 # python-indent: 4