lisp/ob-lilypond.el: Mention epstopdf as a dependency

Thanks to Thomas Morgan for suggesting this.

lisp/ob-lilypond.el

@@ -28,6 +28,8 @@
 ;;
 ;; Lilypond documentation can be found at
 ;; http://lilypond.org/manuals.html
+;;
+;; This depends on epstopdf --- See http://www.ctan.org/pkg/epstopdf.
 
 ;;; Code:
 (require 'ob)