Add docs badge

README.md

@@ -4,6 +4,7 @@
 [![License: MIT](https://img.shields.io/badge/License-MIT-green.svg)](https://opensource.org/licenses/MIT)
 ![](https://github.com/cybertec-postgresql/pg_timetable/workflows/Go%20Build%20&%20Test/badge.svg)
 [![Coverage Status](https://coveralls.io/repos/github/cybertec-postgresql/pg_timetable/badge.svg?branch=v4_dev&service=github)](https://coveralls.io/github/cybertec-postgresql/pg_timetable?branch=v4_dev)
+[![Documentation Status](https://readthedocs.org/projects/pg-timetable/badge/?version=v4_dev)](https://pg-timetable.readthedocs.io/en/v4_dev/?badge=v4_dev)
 [![Release](https://img.shields.io/github/release/cybertec-postgresql/pg_timetable.svg)](https://github.com/cybertec-postgresql/pg_timetable/releases/latest)
 [![Github All Releases](https://img.shields.io/github/downloads/cybertec-postgresql/pg_timetable/total?style=flat-square)](https://github.com/cybertec-postgresql/pg_timetable/releases)
 [![Docker Pulls](https://img.shields.io/docker/pulls/cybertecpostgresql/pg_timetable)](https://hub.docker.com/r/cybertecpostgresql/pg_timetable)