chore(readme): release cadence, modular package stability (#4199)

Author: kuhe

README.md

@@ -42,6 +42,8 @@ visit our [code samples repo](https://github.com/aws-samples/aws-sdk-js-tests).
    1. [Middleware Stack](#middleware-stack)
 1. [Install from Source](#install-from-source)
 1. [Giving feedback and contributing](#giving-feedback-and-contributing)
+1. [Release Cadence](#release-cadence)
+1. [Stability of Modular Packages](#stability-of-modular-packages)
 1. [Known Issues](#known-issues)
    1. [Functionality requiring AWS Common Runtime (CRT)](#functionality-requiring-aws-common-runtime-crt)
 
@@ -507,6 +509,30 @@ client.middlewareStack.add(middleware, {
 
 For a full middleware stack deep dive please check out our [blog post](https://aws.amazon.com/blogs/developer/middleware-stack-modular-aws-sdk-js/).
 
+## Release Cadence
+
+Our releases usually happen once per weekday. Each release increments the 
+minor version, e.g. 3.200.0 -> 3.201.0.
+
+## Stability of Modular Packages
+
+|Package name|containing folder|API controlled by|stability|
+|---|---|---|---|
+|@aws-sdk/client-* Commands|clients|AWS service teams|public/stable|
+|@aws-sdk/client-* Clients|clients|AWS SDK JS team|public/stable|
+|@aws-sdk/lib-*|lib|AWS SDK JS team|public/stable|
+|@aws-sdk/*-signer|packages|AWS SDK JS team|public/stable|
+|@aws-sdk/middleware-stack|packages|AWS SDK JS team|public/stable|
+|remaining @aws-sdk/*|packages|AWS SDK JS team|internal|
+
+Additional notes:
+- internal does not mean a package or interface is constantly changing
+or being actively worked on. It means it is subject to change without any
+notice period. The changes are included in the release notes.
+- public interfaces such as client configuration are also subject to change
+ in exceptional cases. We will try to undergo a deprecation period with 
+ an advance notice.
+
 ## Known Issues
 
 ### Functionality requiring AWS Common Runtime (CRT)