Updates extension readme with Step Functions (#987)

Authored-by: Alan Bogusiewicz <[email protected]>

Updated extension-readme.md file with Step Functions section.

Author: alanbo

extension-readme.md

@@ -50,6 +50,8 @@ These include the following:
 
 -   Amazon Elastic Container Service (Amazon ECS) task definition files
 
+-   Amazon Step Functions state machines
+
 See [Features](#features) below for high-level details, or jump right into the [_AWS Toolkit for Visual Studio Code User Guide_](https://docs.aws.amazon.com/console/toolkit-for-vscode/welcome).
 
 { [Return to Contents](#contents) } or { [Return to Top](#top) }
@@ -84,6 +86,7 @@ For complete setup instructions for these three components, see [Installing the
 -   [AWS CDK Applications](#cdk-apps)
 -   [Amazon EventBridge Schemas](#eventbridge)
 -   [Amazon ECS Task Definition Files](#ecs-files)
+-   [Amazon Step Functions](#sfn-files)
 
 { [Return to Contents](#contents) } or { [Return to Top](#top) }
 
@@ -171,6 +174,24 @@ For full details, see [Working with Amazon Elastic Container Service](https://do
 
 ---
 
+## <a id="sfn-files"></a>Amazon Step Functions
+
+The AWS Toolkit provides support for [Amazon Step Functions](https://docs.aws.amazon.com/step-functions). With the Toolkit installed in VS Code, working with state machines is a more streamlined process.
+
+-   Create, update, execute, and download state machines.
+-   See live graph visualizations of your state machine.
+-   Take advantage of features such as code completion and validation, and code snippets.
+
+Example:
+
+![Amazon Step Functions](./resources/marketplace/sfn-state-machine.png)
+
+For full details, see [Working with AWS Step Functions](https://docs.aws.amazon.com/toolkit-for-vscode/latest/userguide/stepfunctions.html) in the Toolkit's user guide.
+
+{ [Return to Features](#features) } or { [Return to Top](#top) }
+
+---
+
 # <a id="aws-commands"></a>Appendix A: AWS Commands
 
 The AWS Toolkit has several features that you can access through the [Command Palette](#open-command-palette) (select **View**, then **Command Palette**):