Return the path to the mpackage Readme file

Property definitions

nitc $ ReadmeMetric :: readme_path
	# Return the path to the `mpackage` Readme file
	var readme_path: nullable String is lazy do
		var package_path = self.package_path
		if package_path == null then return null
		return package_path.filename / "README.md"
	end
src/metrics/readme_metrics.nit:183,2--188,4