Update slack link (#347)

Signed-off-by: Coline Piloquet <[email protected]>

README.md

@@ -5,6 +5,7 @@
 [![Quality Gate](https://sonarcloud.io/api/project_badges/measure?project=com.powsybl%3Apowsybl-iidm4cpp&metric=alert_status)](https://sonarcloud.io/dashboard?id=com.powsybl%3Apowsybl-iidm4cpp)
 [![MPL-2.0 License](https://img.shields.io/badge/license-MPL_2.0-blue.svg)](https://www.mozilla.org/en-US/MPL/2.0/)
 [![Join the community on Spectrum](https://withspectrum.github.io/badge/badge.svg)](https://spectrum.chat/powsybl)
+[![Slack](https://img.shields.io/badge/slack-powsybl-blueviolet.svg?logo=slack)](https://join.slack.com/t/powsybl/shared_invite/zt-rzvbuzjk-nxi0boim1RKPS5PjieI0rA)
 
 ## Table of contents
 - [Requirements](#requirements)