add some comments about how to test the rendered images

Author: wxiaoguang

modules/badge/badge_glyph_width.go

@@ -7,6 +7,12 @@ import "sync"
 
 // DejaVuGlyphWidthData is generated by `sfnt.Face.GlyphAdvance(nil, <rune>, 11, font.HintingNone)` with DejaVu Sans
 // v2.37 (https://github.com/dejavu-fonts/dejavu-fonts/releases/download/version_2_37/dejavu-sans-ttf-2.37.zip).
+//
+// Fonts defined in "DefaultFontFamily" all have similar widths (including "DejaVu Sans"),
+// and these widths are fixed and don't seem to change.
+//
+// A devtest page "/devtest/badge-actions-svg" could be used to check the rendered images.
+
 var DejaVuGlyphWidthData = sync.OnceValue(func() map[rune]uint8 {
 	return map[rune]uint8{
 		32:  3,