docs: add in why we dropped CJS

website/docs/upgrade/4-5.md

@@ -14,6 +14,14 @@ Version 5.x of Middy no longer supports Node.js versions 16.x. You are highly en
 - Update all errors to be consistent `new Error('message', { cause: { package:'@middy/***', data:*** } })`
 - If using multiple `http-*-body-parsers` on the same endpoint you'll need to set `disableContentTypeError:true`
 
+## Why we deprecated CJS
+
+1. ESM has been well supported in Lambda for almost 2 years now
+2. ESM is almost 2x faster than CJS at p95 [Using Node.js ES modules and top-level await in AWS Lambda](https://aws.amazon.com/blogs/compute/using-node-js-es-modules-and-top-level-await-in-aws-lambda/)
+3. Maintainability; Maintaining a package to work with every transpilers and build tools that are constantly changing over time is hard and time consuming.
+
+If you're not able to upgrade your codebase to using ESM yet, that's okay, v4.x is super stable and support CJS.
+
 ## Core
 
 - Updated `plugin.timeoutEarlyResponse(...)` to throw new error with name `TimeoutError` to match new `AbortSignal.timeout()`.