doc: adding cue community contact information

adds invite link for cuelang.slack.com workspace to the documentation
this is useful for new commers to interact with others using CUE

Issue #63

Change-Id: Ibb8086db760fcc5871c838b9ac41d15bac803e8c
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2660
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: xinau

README.md

@@ -137,6 +137,17 @@ To report issues or make a feature request, use the
 
 Changes can be contributed using Gerrit or Github pull requests.
 
+
+### Contact
+
+You can get in touch with the cuelang community in the following ways:
+
+- Chat with us on our
+  [Slack workspace](https://join.slack.com/t/cuelang/shared_invite/enQtNjk1NzU1MzMwMjI2LWQ0ZDA2OTRjYTQ0MGUyZDBiYWIzZjVhZGI1YWVlNTVmMWU3MTFhY2MxZmRmMjBiMTA5MGQ1YzUyMmMzN2IxYzc).
+
+
+---
+
 Unless otherwise noted, the CUE source files are distributed
 under the Apache 2.0 license found in the LICENSE file.