diff --git a/asciidoctor/extract-header-ids b/asciidoctor/extract-header-ids
index 1eaeba1..896e08a 100755
--- a/asciidoctor/extract-header-ids
+++ b/asciidoctor/extract-header-ids
@@ -1,5 +1,7 @@
 #!/usr/bin/env ruby
 
+# https://github.com/cirosantilli/linux-kernel-module-cheat#asciidoctor-extract-header-ids
+
 require 'asciidoctor'
 require 'asciidoctor/extensions'
 require 'pry'