doc(codepipeline): update readme

packages/@aws-cdk/aws-codepipeline-actions/README.md

@@ -997,3 +997,31 @@ pipeline.addStage({
 
 See [the AWS documentation](https://docs.aws.amazon.com/codepipeline/latest/userguide/action-reference-StepFunctions.html)
 for information on Action structure reference.
+
+### AWS Stack Sets
+
+This module contains an Action that allows you to create/update a StackSet in a Pipeline:
+
+```ts
+const pipeline = new codepipeline.Pipeline(this, 'MyPipeline');
+const inputArtifact = new codepipeline.Artifact();
+const stepFunctionAction = new codepipeline_actions.CloudFormationStackSetAction({
+  actionName: 'UpdateStackSet',
+  stackSetName: 'MyStackSet',
+  deploymentTargets: ['111111111111', '2222222222'],
+  regions: ['us-east-1', 'us-west-1'],
+  templatePath: inputArtifact.atPath('container.yaml'),
+  parametersPath: inputArtifact.atPath('parameters.json'),
+  cfnCapabilities: [
+    cdk.CfnCapabilities.NAMED_IAM,
+    cdk.CfnCapabilities.AUTO_EXPAND,
+  ],
+});
+pipeline.addStage({
+  stageName: 'Deploy',
+  actions: [stepFunctionAction],
+});
+```
+
+See [the AWS documentation](https://docs.aws.amazon.com/codepipeline/latest/userguide/action-reference-StackSets.html)
+for information on Action structure reference.