doc(redshift): add details on cluster resizing

packages/@aws-cdk/aws-redshift-alpha/README.md

@@ -543,3 +543,19 @@ const cluster = new Cluster(this, 'Redshift', {
 });
 cluster.addIamRole(role);
 ```
+
+## Resizing
+
+As your data warehousing needs change, it's possible to resize your Redshift cluster, but it's important to understand certain caveats.
+Most importantly, if the cluster was deployed via CDK, the cluster should be resized via CDK so it's registered in the AWS CloudFormation template.
+This ensures the `NumberOfNodes` parameter gets updated so the correct type of resize is done. There are two types of resize operations:
+
+* Elastic resize - You can add nodes to or remove nodes from the cluster. Also, you can change the node type, such as from DS2 nodes to RA3 nodes.
+AWS recommmends elastic resize as the first option, as it's a fast operation and typically completes in minutes. Elastic resize is appropriate when you:
+  * Add or reduce nodes in your existing cluster, but don't change the node type.
+  * Change the node type of your cluster.
+
+* Classic resize - Number of nodes, node type, or both, can be changed. This operation takes longer to complete, but is useful when the resize operation
+doesn't meet the criteria of an elastic resize. See [AWS Knowledge Center](https://repost.aws/knowledge-center/redshift-troubleshoot-elastic-resize) for further conditions that require classic resizing.
+
+See the [AWS Redshift documentation](https://docs.aws.amazon.com/redshift/latest/mgmt/managing-cluster-operations.html#rs-resize-tutorial) for full details on Redshift cluster resizing.