Added contributing information

CONTRIBUTING.md

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+# Contributing
+
+Contributions are **welcome**, will be fully **credited** and much **appreciated**.
+
+You may contribute via Pull Requests or Issues on [Github](https://github.com/RonRademaker/tactician-scheduler-plugin).
+
+## Pull Requests
+
+- **Travis** checks all pull request and everything must pass.
+
+- **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)** - The easiest way to apply the conventions is to install [PHP Code Sniffer](http://pear.php.net/package/PHP_CodeSniffer). Travis also checks this.
+
+- **Add tests** - to ensure a high quality code base and prevent regression. Code coverage must stay at 100%, use ignore statements to avoid stupid tests or indicate untestable lines.
+
+- **One pull request per feature** - If you want to do more than one thing, send multiple pull requests.
+
+- **Be respectful** - Be excellent to other contributors.
+
+## My Promise
+
+I won't promise you I accept all Pull Requests that match the criteria set out above. I do promise you I'll try to help you to fix your problem if your PR doesn't align with my vision for this package or to give feedback on what you created to let it evolve to a quality I believe is good enough.