Add README for CodePipeline sample source app

README.md

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+# CodePipeline Sample Source — Message Utility Java App
+
+This repository is a sample source application used to demonstrate [LocalStack's AWS CodePipeline](https://docs.localstack.cloud/aws/services/codepipeline/) emulation. It showcases a full CI/CD workflow with CodeBuild (Maven build + JUnit tests), CodeDeploy (ECS blue/green deployment), and CloudFormation stack provisioning.
+
+## Overview
+
+The application is a simple Java utility (`MessageUtil`) built with Maven and packaged as a JAR. The pipeline:
+
+1. **Source** — pulls this repository as the pipeline source.
+2. **Build** — uses CodeBuild with Amazon Corretto 11 to compile the Maven project and run JUnit tests, producing `target/messageUtil-1.0.jar`.
+3. **Deploy** — deploys an nginx-based Docker image (serving `index.html`) to an ECS service via CodeDeploy blue/green deployment.
+4. **Provision** — optionally creates an SQS queue via the included CloudFormation template.
+
+## Repository Structure
+
+```bash
+.
+├── Dockerfile          # nginx image serving index.html
+├── appspec.yml         # CodeDeploy AppSpec for ECS blue/green deployment
+├── buildspec.yml       # CodeBuild build specification (Java/Maven)
+├── cfn_stack.yaml      # CloudFormation template — creates an SQS queue
+├── index.html          # Static HTML page served by nginx
+├── pom.xml             # Maven project descriptor for Message Utility
+└── src/
+    ├── main/java/MessageUtil.java       # Java utility class
+    └── test/java/TestMessageUtil.java   # JUnit tests
+```
+
+## Prerequisites
+
+- A valid [LocalStack for AWS license](https://localstack.cloud/pricing), which provides a [`LOCALSTACK_AUTH_TOKEN`](https://docs.localstack.cloud/getting-started/auth-token/) required to run these samples.
+- [Docker](https://docs.docker.com/get-docker/) with access to the Docker socket.
+- - AWS CLI and [`awslocal`](https://github.com/localstack/awscli-local)
+
+```bash
+export LOCALSTACK_AUTH_TOKEN=<your-auth-token>
+```
+
+## Usage
+
+This repository is intended to be used as the **source stage** of a LocalStack CodePipeline sample. Refer to the parent pipeline repository for end-to-end setup instructions.
+
+When used standalone, you can build the Maven project locally:
+
+```bash
+mvn install
+```
+
+This compiles `MessageUtil.java`, runs the JUnit tests, and produces `target/messageUtil-1.0.jar`.
+
+## License
+
+Licensed under the Apache License 2.0. See [LICENSE](LICENSE).