readme: make local file links work relative to out/

Thanks to awesome help at:
https://github.com/asciidoctor/asciidoctor/issues/3324

asciidoctor/extract-link-targets

@@ -0,0 +1,20 @@
+#!/usr/bin/env ruby
+
+# https://github.com/cirosantilli/linux-kernel-module-cheat#asciidoctor-extract-links
+
+require 'asciidoctor'
+require 'asciidoctor/extensions'
+
+class Main < Asciidoctor::Extensions::InlineMacroProcessor
+  use_dsl
+  named :link
+  def process parent, target, attrs
+    puts target
+  end
+end
+
+Asciidoctor::Extensions.register do
+  inline_macro Main
+end
+
+(Asciidoctor.load_file(ARGV[0])).convert