Add note about Doxygen to the TODO list.

TODO.md

@@ -33,6 +33,7 @@ G-code or SVG. This would be a big change to the library and would
 introduce a lot of new code. It would also be a big testing issue.
 * Add circles and arcs to the PenDownDistance and plot time calculations.
 * Make plot time take into account pen speed if '-v' is used.
+* Add Doxygen comments to the library source.
 
 ## Turtle library
 * Implement run-time option setting in all Turtle plots, and in the
@@ -41,6 +42,7 @@ direct HPGL programs and Turtle programs.
 * Make Turtle library drive the plotter directly, as the HPGL library
 does. This would eliminate the need to redirect the output to the
 plotter (using shell I/O redirection) when plotting Turtle programs.
+* Add Doxygen comments to the library source.
 
 ## Plotting programs
 * Add new drawings! Especially colouring-in drawings, all-over patterns,
@@ -54,3 +56,4 @@ twist, zigzag.
 * Clean up the various 'dala' drawings. Eliminate the 'dala2c' variant
 by adding colour to 'dala2.c'.
 * Add run-time variation to 'dala2.c'.
+* Consider using Doxygen to document the programs.