docs(readme): announce v3 api-reference (#4400)

developer preview

Author: MYoung25

README.md

@@ -15,6 +15,8 @@ or [API Reference](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/index.
 
 For release notes, see the [CHANGELOG](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md). Prior to v2.4.8, release notes can be found at https://aws.amazon.com/releasenotes/?tag=releasenotes%23keywords%23javascript
 
+### We are excited to announce the [developer preview](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/preview/) of AWS SDK for JavaScript v3's new API documentation.
+
 ## Version 2.x Support
 We are formalizing our plans to enter **AWS SDK for JavaScript v2** into maintenance mode in 2023.