Add details on how to get Slack invite to docs (#329)

* Update contributing.md

* Right not left

Author: mattbaileyuk

docs/contributing.md

@@ -4,9 +4,15 @@ title: Contributing to JSONata
 sidebar_label: Community and Contributing
 ---
 
-## General questions - StackOverflow
+## General questions
 
-See questions [tagged with `jsonata`](https://stackoverflow.com/questions/tagged/jsonata)
+### StackOverflow
+
+A good first start would be to ask a question here, or see previous questions [tagged with `jsonata`](https://stackoverflow.com/questions/tagged/jsonata)
+
+### Slack
+
+There are a community of users in the [JSONata Slack workspace](http://jsonata.slack.com) who can help with queries. You can request an invite by going to the [JSONata exerciser](http://try.jsonata.org/) and clicking the Slack icon in the top-right.
 
 ## Bugs reporting