use shared-invite url for slack

Author: usefulthink

README.md

@@ -19,7 +19,7 @@ The aim of the project is to create an easy to use, lightweight, 3D library with
 [Questions](http://stackoverflow.com/questions/tagged/three.js) —
 [Forum](https://discourse.threejs.org/) —
 [Gitter](https://gitter.im/mrdoob/three.js) —
-[Slack](https://threejs-slack.herokuapp.com/)
+[Slack](https://join.slack.com/t/threejs/shared_invite/enQtMzYxMzczODM2OTgxLTQ1YmY4YTQxOTFjNDAzYmQ4NjU2YzRhNzliY2RiNDEyYjU2MjhhODgyYWQ5Y2MyZTU3MWNkOGVmOGRhOTQzYTk)
 
 ### Usage ###