fix(cx-api): adding missing readme (#34003)

### Issue #

Closes #34006

### Reason for this change

Readme was not added to #33983

### Description of changes

Adding readme based on feature description

https://github.com/aws/aws-cdk/blob/b5774195969a83d92323c854f3d7a1930e51e23d/packages/aws-cdk-lib/cx-api/README.md?plain=1#L698-L714

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: IkeNefcy

packages/aws-cdk-lib/cx-api/README.md

@@ -694,3 +694,21 @@ _cdk.json_
   }
 }
 ```
+
+* `@aws-cdk/cognito:logUserPoolClientSecretValue`
+
+When this feature flag is enabled, the SDK API call response to desribe user pool client values will be logged in the custom 
+resource lambda function logs.
+
+When this feature flag is disabled, the SDK API call response to describe user pool client values will not be logged in the custom 
+resource lambda function logs. 
+
+_cdk.json_
+
+```json
+{
+  "context": {
+    "@aws-cdk/cognito:logUserPoolClientSecretValue": true
+  }
+}
+```