docs(aws-sam): reference related modules in README (#2502)

Add references to reference documentation of related CDK modules.

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

@@ -1,7 +1,17 @@
 ## AWS Serverless Application Model Construct Library
 
+This module includes low-level constructs that synthesize into `AWS::Serverless` resources.
+
 ```ts
 const sam = require('@aws-cdk/aws-sam');
 ```
 
-Note: we recommended to use `@aws-cdk/aws-lambda`, `aws-cdk/aws-lambda-event-sources` and `@aws-cdk/aws-apigateway` packages to build 'serverless' applications with the CDK instead of the `AWS::Serverless` resources exposes by `@aws-cdk/aws-sam`.
+### Related
+
+The following AWS CDK modules include constructs that can be used to work with Amazon API Gateway and AWS Lambda:
+
+* [aws-lambda](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-lambda-readme.html): define AWS Lambda functions
+* [aws-lambda-event-sources](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-lambda-event-sources-readme.html): classes that allow using various AWS services as event sources for AWS Lambda functions
+* [aws-apigateway](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-apigateway-readme.html): define APIs through Amazon API Gateway
+* [aws-codedeploy](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-codedeploy-readme.html#lambda-applications): define AWS Lambda deployment with traffic shifting support
+