[readme] Add Slack channel invite link
@@ -95,6 +95,7 @@ Links
- [Project homepage](http://owtf.github.io/)
- [IRC](http://webchat.freenode.net/?randomnick=1&channels=%23owtf&prompt=1&uio=MTE9MjM20f)
- [Wiki](https://www.owasp.org/index.php/OWASP_OWTF)
+- [Slack](https://owasp.herokuapp.com) and join channel `#project-owtf`
- [User Documentation](http://docs.owtf.org/en/latest/)
- [Youtube channel](https://www.youtube.com/user/owtfproject)
- [Slideshare](http://www.slideshare.net/abrahamaranguren/presentations)
