Update new exercise guide to mention version file

CONTRIBUTING.md

 The `build.gradle` file can just be copied from any other exercise submodule.
 See the [POLICIES doc](https://github.com/exercism/java/blob/master/POLICIES.md#starter-implementations) for an explanation of when you need to add a starter implementation.
 
+4. Check if there is canonical data for the exercise you're adding.
+This can be found at `https://github.com/exercism/problem-specifications/tree/master/exercises/EXERCISE-SLUG/canonical-data.json`.
+If there is canonical data for your exercise then you should follow this when making the tests.
+We aim to follow the canonical data as closely as possible in our tests to ensure thorough test coverage.
+If there is canonical data available you also need to create a file at `exercises/exercise-slug/.meta/version` specifying the canonical data version (e.g. `1.0.0`).
+The canonical data version can be found at the top of the canonical data file for that exercise.
+See other exercises, e.g. [acronym](https://github.com/exercism/java/tree/master/exercises/acronym/.meta), for an example `version` file.
+
 Hopefully that should be enough information to help you port an exercise to the Java track.
 Feel free to open an issue or post in the [Gitter exercism/java room](https://gitter.im/exercism/java) if you have any questions and we'll try and answer as soon as we can.