doc: update DESIGN.md to include an Overview and entry point for SAM (#…

…370)

* Update DESIGN.md to include an Overview and entry point for SAM

* Updated wording in DESIGN.md based on PR feedback

DESIGN.md

@@ -1,3 +1,11 @@
+# Overview
+
+SAM is called by the CloudFormation Service. CloudFormation recognises the `Transform: AWS::Serverless-2016-10-31` header and invokes the SAM translator. This will then take your SAM template and expand it
+into a full fledged CloudFormation Template. The CloudFormation Template that is produced from SAM is the template that is executed by CloudFormation to create/update/delete AWS resources.
+
+The entry point for SAM starts in the Translator class [here](https://github.com/awslabs/serverless-application-model/blob/develop/samtranslator/translator/translator.py#L29), where SAM iterates through the
+template and acts on `AWS::Serverless::*` Type Resources.
+
 # Design decisions
 
 Document design decisions here.