docs(redshift): add details on cluster resizing (#26928)

I felt it best to keep most of the details in the linked documentation, but I believe this provides a good overview.

Closes #21103.

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Author: msambol

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

@@ -543,3 +543,28 @@ const cluster = new Cluster(this, 'Redshift', {
 });
 cluster.addIamRole(role);
 ```
+
+## Resizing
+
+As your data warehousing needs change, it's possible to resize your Redshift cluster. If the cluster was deployed via CDK, 
+it's important to resize it via CDK so the change is registered in the AWS CloudFormation template. 
+There are two types of resize operations:
+
+* Elastic resize - Number of nodes and node type can be changed, but not at the same time. Elastic resize is the default behavior, 
+as it's a fast operation and typically completes in minutes. Elastic resize is only supported on clusters of the following types:
+  * dc1.large (if your cluster is in a VPC)
+  * dc1.8xlarge (if your cluster is in a VPC)
+  * dc2.large
+  * dc2.8xlarge
+  * ds2.xlarge
+  * ds2.8xlarge
+  * ra3.xlplus
+  * ra3.4xlarge
+  * ra3.16xlarge
+
+* 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. If you prefer classic resizing,
+you can set the `classicResizing` flag when creating the cluster.
+
+There are other constraints to be aware of, for example, elastic resizing does not support single-node clusters and there are 
+limits on the number of nodes you can add to a cluster. See the [AWS Redshift Documentation](https://docs.aws.amazon.com/redshift/latest/mgmt/managing-cluster-operations.html#rs-resize-tutorial) and [AWS API Documentation](https://docs.aws.amazon.com/redshift/latest/APIReference/API_ResizeCluster.html) for more details.