Added project-root var resolution in readme generation script

docs/src/main/ruby/generate_readme.sh

@@ -17,7 +17,10 @@ end
 file = ARGV[0] if ARGV.length>0
 
 # Copied from https://github.com/asciidoctor/asciidoctor-extensions-lab/blob/master/scripts/asciidoc-coalescer.rb
-doc = Asciidoctor.load_file file, safe: :unsafe, header_only: true, attributes: options[:attributes]
+attributes = {}
+attributes['project-root'] = File.expand_path("#{base_dir}/../")
+
+doc = Asciidoctor.load_file file, safe: :unsafe, header_only: true, attributes: attributes
 header_attr_names = (doc.instance_variable_get :@attributes_modified).to_a
 header_attr_names.each {|k| doc.attributes[%(#{k}!)] = '' unless doc.attr? k }
 attrs = doc.attributes