Add README

packages/@aws-cdk/aws-cognito/README.md

@@ -328,4 +328,25 @@ const awesomePool = UserPool.fromUserPoolId(stack, 'awesome-user-pool', 'us-east
 
 const otherAwesomePool = UserPool.fromUserPoolArn(stack, 'other-awesome-user-pool',
   'arn:aws:cognito-idp:eu-west-1:123456789012:userpool/us-east-1_mtRyYQ14D');
-```
+```
+
+### App Clients
+
+An app is an entity within a user pool that has permission to call unauthenticated APIs (APIs that do not have an
+authenticated user), such as APIs to register, sign in, and handle forgotten passwords. To call these APIs, you need an
+app client ID and an optional client secret. Read [Configuring a User Pool App
+Client](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-settings-client-apps.html) to learn more.
+
+The following code creates an app client and retrieves the client id -
+
+```ts
+const pool = new UserPool(this, 'Pool');
+
+const client = new UserPoolClient(stack, 'Client', {
+  userPool: pool
+});
+
+const clientId = client.userPoolClientId; 
+```
+
+Existing app clients can be imported into the CDK app using the `UserPoolClient.fromUserPoolClientId()` API.