Minor update to README.

README.md

@@ -2,11 +2,14 @@
 
 Vector graphics demo software to run on an HPGL pen plotter.
 
-The programs are in C and may be compiled with 'gcc' on Linux.
-They generate HPGL and send it to a plotter via the parallel port,
-/dev/usb/lp0 or to a file.
+The programs are in C and may be compiled with 'gcc' on Linux
+(MacOS X may also work).
+They generate HPGL and send it to a plotter via the parallel port
+(/dev/usb/lp0),
+the serial port (/dev/ttyUSB0),
+or to a file.
 Most of the programs accept command-line arguments to scale the vector
-drawings up or down (default size is suitable for A3 paper).
+drawings up or down (default size is suitable for ISO A3 paper).
 
 ## HPGL
 
@@ -27,14 +30,16 @@ Install it with a command-line such as:
 sudo apt-get install hp2xx
 
 The Makefile will use 'hp2xx' to generate low-resolution preview images
-for the plots.
+in PNG format.
+It also converts the HPGL to SVG for previewing in a browser or for
+plotting on newer types of plotter such as the AxiDraw.
 
 ## The Plotters
 
 Most of the programs here have been tested on a Roland DXY-990 A3
 flat-bed pen plotter.
 Some have also been used at A1 size on the much bigger Roland DPX-3300.
-A few have been tested on the HP 7550 A3 grit-wheel plotter.
+A few have been tested on the fast HP 7550A A3 grit-wheel plotter.
 
 ## The Pens
 
@@ -44,6 +49,8 @@ that fit the Roland plotters.
 Other types of pen will no doubt work, and few of the plots here rely
 on special line width or other pen properties.
 
+Most of the plots require only a single pen.
+
 None of the drawings are intended for use with vinyl cutters, but there's
 plenty of scope to try that sort of thing.