Create CONTRIBUTING.md

Author: danmichaelo

CONTRIBUTING.md

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+Thanks for considering contributing to this project! Pull requests are welcome,
+and if you want to discuss something before starting (or just check if the maintainer is still alive),
+feel free to open an issue first.
+
+## Code style
+
+This project comes with a [ruleset.xml](https://github.com/scriptotek/php-marc/blob/master/ruleset.xml) file that
+defines the code style (which is PSR-2 at the time of this writing), so it can be checked with
+[phpcs](https://github.com/squizlabs/PHP_CodeSniffer):
+
+    phpcs --standard=ruleset.xml src
+
+The project also comes with an [.overcommit.yml](https://github.com/scriptotek/php-marc/blob/master/.overcommit.yml)
+file so you can use [Overcommit](https://github.com/sds/overcommit)'s Git hooks to have your changes checked before
+each commit.
+
+## Tests
+
+Tests are run using [PhpUnit](https://phpunit.de/):
+
+    ./vendor/bin/phpunit
+
+If you add new functionality, please consider also adding a test case for it.
+
+## Changelog
+
+Consider adding an entry to the [CHANGELOG](https://github.com/scriptotek/php-marc/blob/master/CHANGELOG.md) as part
+of your commit.
+
+## Code of conduct
+
+This project adheres to the [Open Code of Conduct][code-of-conduct]. By
+participating, you are expected to honor this code.
+
+[code-of-conduct]: https://github.com/civiccc/code-of-conduct