added README file

org.eclipse.xtend.maven.archetype/README.md

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+To run the integration tests, the `maven.home` variable must defined, i.e., passed to the Maven build with `-Dmaven.home=...`.
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+For example, if the Maven executable is `/usr/local/bin/mvn`, you need to pass `-Dmaven.home=/usr/local` (i.e., without the `bin` subdirectory).
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+The Maven Invoker Plugin's goal `install` is attached to the `pre-integration-test` phase, in order to install the needed Maven artifacts of Xtext/Xtend into the `target/.m2test` folder, which will be then used by the integration test to build the test project generated from the archetype.