chore: removing partitions.io board and adding new slack channel in R…

…EADME (aws#10411)

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removing the partitions.io board link and replacing with the cdk.dev slack channel.


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

README.md

@@ -134,7 +134,7 @@ You may also find help on these community resources:
   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)
-* Check out the [partitions.io board](https://partitions.io/cdk)
+* A community-driven Slack channel is also available, invite at [cdk.dev](https://cdk.dev)
 
 ### Roadmap