Add link to the Gitter chat room.

And rephrase the Resources section in general.

Author: sjrd

index.md

@@ -47,9 +47,10 @@ and not part of any of their support contracts. You have been warned!</p>
 
 ## Resources
 
-* [Documentation](./doc/)
-* [Official mailing list](https://groups.google.com/forum/?fromgroups#!forum/scala-js)
-* [Scala.js tag on Stackoverflow](http://stackoverflow.com/questions/tagged/scala.js). Please help building it up by asking questions.
+* [Documentation](./doc/): APIs, guides, and reference.
+* [Scala.js tag on Stackoverflow](http://stackoverflow.com/questions/tagged/scala.js): for delimited technical questions on using Scala.js.
+* [Official mailing list](https://groups.google.com/forum/?fromgroups#!forum/scala-js): for general discussion, ideas, announcements of your libraries, etc.
+* [Gitter chat room](https://gitter.im/scala-js/scala-js): come and meet the community of Scala.js users.
 
 Presentations: