rgbgfx: Add an option to take the CGB's color profile into account

src/gfx/rgbgfx.1

@@ -61,6 +61,8 @@ Same as
 but the attrmap file output name is made by taking the input filename, removing
 the file extension, and appending
 .Pa .attrmap .
+.It Fl C
+Use the color curve of the Game Boy Color when generating palettes.
 .It Fl D
 Debug features are enabled.
 .It Fl f