Create CONTRIBUTING.md

CONTRIBUTING.md

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+Hi! Thanks for contributing to the Exercism Java track!
+
+Before opening your pull request, please review the track policies linked below and make sure your changes comply with them all. This helps us focus our review time on the more important aspects of your changes.
+
+- [Prefer instance methods](https://github.com/exercism/java/blob/master/POLICIES.md#prefer-instance-methods)
+- [Starter implementations](https://github.com/exercism/java/blob/master/POLICIES.md#starter-implementations)
+- [Ignore noninitial tests](https://github.com/exercism/java/blob/master/POLICIES.md#ignore-noninitial-tests)
+- [Multiple file submissions](https://github.com/exercism/java/blob/master/POLICIES.md#multiple-file-submissions)