Add README.md content

Author: gilesgas

README.md

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 # simple-java-maven-app
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+This repository is for the [Using Jenkins to build a Java/Maven project]( https://jenkins.io/doc/tutorials/using-jenkins-to-build-a-java-maven-project/) tutorial in the [Jenkins User Documentation](https://jenkins.io/doc/).
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+The repository contains a simple Java application which outputs the string "Hello world!" and is accompanied by a unit test to check that the main application works as expected.
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+The `jenkins` directory contains an example of the `Jenkinsfile` (i.e. Pipeline) you'll be creating yourself during the tutorial and the `scripts` directory contains a shell script with commands that are executed when Jenkins processes the "Deliver" stage of your Pipeline.