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

asciidoctor/link-target-up.rb

@@ -0,0 +1,31 @@
+#!/usr/bin/env ruby
+
+=begin
+README.html links break because we place that output file in out/
+This extension hacks local link targets to the right path.
+=end
+
+require 'asciidoctor'
+require 'asciidoctor/extensions'
+
+class Main < Asciidoctor::Extensions::InlineMacroProcessor
+  use_dsl
+  named :link
+  ExternalLinkRegex = /^https?:\/\//
+  def process parent, target, attrs
+    text = attrs[1]
+    if text.nil? || text.empty?
+      text = target
+    end
+    if !ExternalLinkRegex.match?(target)
+      target = File.join('..', target)
+    end
+    create_anchor parent, text, type: :link, target: target
+  end
+end
+
+Asciidoctor::Extensions.register do
+  inline_macro Main
+end
+
+#(Asciidoctor.load_file(ARGV[0])).convert