docs(README): Add coding standards badge

Author: julienloizelet

README.md

@@ -7,6 +7,7 @@
 ![Version](https://img.shields.io/github/v/release/crowdsecurity/php-cs-bouncer?include_prereleases)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=crowdsecurity_php-cs-bouncer&metric=alert_status)](https://sonarcloud.io/dashboard?id=crowdsecurity_php-cs-bouncer)
 [![Test suite](https://github.com/crowdsecurity/php-cs-bouncer/actions/workflows/test-suite.yml/badge.svg)](https://github.com/crowdsecurity/php-cs-bouncer/actions/workflows/test-suite.yml)
+[![Coding standards](https://github.com/crowdsecurity/php-cs-bouncer/actions/workflows/coding-standards.yml/badge.svg)](https://github.com/crowdsecurity/php-cs-bouncer/actions/workflows/coding-standards.yml)
 ![Licence](https://img.shields.io/github/license/crowdsecurity/php-cs-bouncer)