Add diagram and details to readme (#4)

Author: honglu

README.md

@@ -1,7 +1,26 @@
-## AWS Serverless Codepipeline Serverlessrepo Publish
+## AWS Serverless Codepipeline Serverlessrepo Publish ![Build Status](https://codebuild.us-east-1.amazonaws.com/badges?uuid=eyJlbmNyeXB0ZWREYXRhIjoiRWVJS0x2ZFJGMTJYZWVKWHRxZXQzV3dHYlM4enJPc3k3bno4cmZRMmtwQkR5dGRSYUp1bDF3bnNHUE1NV1JpTHpTWC9KZ1Q4YmhtcG5aOXdNVWd4U2ZBPSIsIml2UGFyYW1ldGVyU3BlYyI6IlVhNGJ2dXlnZG1kbHJLS2siLCJtYXRlcmlhbFNldFNlcmlhbCI6MX0%3D&branch=master)
 
 This is a serverless app that publishes applications to AWS Serverless Application Repository. This app creates a Lambda function that a user could then use as an Invoke action target in their CodePipeline.
 
+## Architecture
+
+![App Architecture](https://github.com/awslabs/aws-serverless-codepipeline-serverless-publish/raw/master/app-architecture.png)
+
+1. App has a single Lambda function ServerlessRepoPublish lambda.
+1. ServerlessRepoPublish lambda is invoked by CodePipeline as part of the Invoke Action of a pipeline.
+1. ServerlessRepoPublish lambda is passed the S3 URL of the packaged SAM template in the CodePipeline S3 bucket.
+1. ServerlessRepoPublish lambda downloads the template and parses its Metadata to get application information for calls to CreateApplication/UpdateApplication. 
+1. ServerlessRepoPublish lambda then does the create or update job processor logic:
+   1. Call [AcknowledgeJob](https://docs.aws.amazon.com/codepipeline/latest/APIReference/API_AcknowledgeJob.html) API to claim the job.
+   1. Read SAM template and parse application metadata.
+   1. Call [CreateApplication](https://docs.aws.amazon.com/serverlessrepo/latest/devguide/applications.html) API with metadata and pass SAM template with semantic version from template metadata.
+   1. If success, call [PutJobSuccessResult](https://docs.aws.amazon.com/codepipeline/latest/APIReference/API_PutJobSuccessResult.html)
+   1. If application already exists
+      1. Call [GetApplication](https://docs.aws.amazon.com/serverlessrepo/latest/devguide/applications-applicationid.html) - Application ARN can be parsed from the 4xx error message. NOTE: This isn't the cleanest solution, but it doesn't require an API change to SAR.
+      1. Call [UpdateApplication](https://docs.aws.amazon.com/serverlessrepo/latest/devguide/applications-applicationid.html) if any metadata has changed
+      1. Call [CreateApplicationVersion](https://docs.aws.amazon.com/serverlessrepo/latest/devguide/applications-applicationid-versions-semanticversion.html) with SAM template. If it already exists, do nothing.
+   1. If API calls fail for any other reason, call [PutJobFailureResult](https://docs.aws.amazon.com/codepipeline/latest/APIReference/API_PutJobFailureResult.html) with failure details.
+
 ## License Summary
 
 This sample code is made available under the MIT license.