docs: update README.md in smithy-aws-typescript (#5316)

* docs: update README.md in smithy-aws-typescript

* docs: update README.md

Author: kuhe

Diff for: codegen/README.md

@@ -2,6 +2,12 @@
 
 This package is used to build each client in the SDK.
 
+The code generation stack is made up of 3 components:
+
+ 1. [Smithy](https://github.com/smithy-lang/smithy) - the core libraries for code generation. They are not intended to contain anything specific to TypeScript or AWS SDKs.
+ 2. [Smithy-TypeScript](https://github.com/awslabs/smithy-typescript) - This uses Smithy to generate code for the TypeScript (JavaScript) language runtimes. While this is maintained by AWS, it is not intended to contain anything specific to AWS.
+ 3. [Smithy-AWS-TypeScript](https://github.com/aws/aws-sdk-js-v3/tree/docs/readme/codegen) - This is the component here in the AWS SDK for JavaScript (v3) repository. It uses the two upstream components, Smithy and Smithy-TypeScript, to generate the AWS SDK for JavaScript (v3). Anything specific to AWS should only be found in this package.
+
 ## Building
 
 By running `./gradlew :sdk-codegen:build`, this package will: