docs: add Slack link to the main ReadMe (aws#8388)

Fixes aws#6669

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

README.md

@@ -118,6 +118,7 @@ for tracking bugs and feature requests.
 * Ask a question on [Stack Overflow](https://stackoverflow.com/questions/tagged/aws-cdk)
   and tag it with `aws-cdk`
 * Come join the AWS CDK community on [Gitter](https://gitter.im/awslabs/aws-cdk)
+* Talk in the CDK channel of the [AWS Developers Slack workspace](https://awsdevelopers.slack.com) (invite required)
 * Open a support ticket with [AWS Support](https://console.aws.amazon.com/support/home#/)
 * If it turns out that you may have found a bug,
   please open an [issue](https://github.com/aws/aws-cdk/issues/new)