Update README.md

add badge for doc generation

README.md

@@ -21,5 +21,6 @@ The various subdirectories
 ![main test](https://github.com/ivoa/vo-dml/actions/workflows/test.yml/badge.svg)
 ![site build](https://github.com/ivoa/vo-dml/actions/workflows/site.yml/badge.svg)
 [![gradle plugin](https://img.shields.io/gradle-plugin-portal/v/net.ivoa.vo-dml.vodmltools?label=gradle%20plugin)](https://plugins.gradle.org/plugin/net.ivoa.vo-dml.vodmltools)
+[![Standard Document PDF Preview generation](https://github.com/ivoa/vo-dml/actions/workflows/std.yml/badge.svg)](https://github.com/ivoa/vo-dml/actions/workflows/std.yml)
 
 _note that this project was moved from https://volute.g-vo.org/svn/trunk/projects/dm/vo-dml/_