Add Slack

Author: bcremer

README.md

@@ -3,6 +3,8 @@
 * Find upcoming meetups on [Meetup](https://www.meetup.com/phpugms/)
 * Follow us on [Twitter](https://twitter.com/phpugms/)
 * Contribute Slides and links on [GitHub](https://github.com/phpugms/)
+* Chat with us in the `#phpugms` room on the [PHPUG Slack](https://phpug.slack.com/) (Get your [Invite](https://php.ug/slackinvite))
+
 
 ## Slides 2017