Update sermon-a4.css

sermon-a4.css

@@ -2,8 +2,13 @@
 author: simonpipe (codeberg.org/simonpipe/)
 License: MIT
 
-This theme builds upon sermon-slide.css and optimizes it for A4 booklet printing. 
+This theme builds upon "sermon-slide.css" and is optimized for A4 printing.
 Inspired by "a4-clean.css" by stanfrdb (https://github.com/stanfrbd/A4-marp/tree/main)
+
+Usage:
+- Default: Standard A4 layout with 12pt font size.
+- Class 'booklet': Scales the layout to 18pt for easy-to-read booklet printing.
+  (Set 'class: booklet' in your Markdown YAML header)
 */
 
 @import "sermon-slide";