fix(lambda):Update readme

fixes aws#8257

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

@@ -266,6 +266,9 @@ The `logRetention` property can be used to set a different expiration period.
 It is possible to obtain the function's log group as a `logs.ILogGroup` by calling the `logGroup` property of the
 `Function` construct.
 
+By default, CDK uses the AWS SDK retry options when creating a log group. The `logRetentionRetryOptions` property 
+allows you to customize the maximum number of retries and base backoff duration.
+
 *Note* that, if either `logRetention` is set or `logGroup` property is called, a [CloudFormation custom
 resource](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-cfn-customresource.html) is added
 to the stack that pre-creates the log group as part of the stack deployment, if it already doesn't exist, and sets the