add link to slack-signup

index.html

@@ -234,6 +234,7 @@
 				<a href="https://stackoverflow.com/questions/tagged/three.js">questions</a><br />
 				<a href="https://discourse.threejs.org/">forum</a><br />
 				<a href="http://webchat.freenode.net/?channels=three.js">chat</a><br />
+				<a href="https://threejs-slack.herokuapp.com">slack</a><br />
 				<a href="https://plus.google.com/+ThreejsOrg">google+</a><br /><br />
 
 				<a href="editor/">editor</a>