SDK-533: Add CONTRIBUTING.md

Author: davidgrayston

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing
+
+After cloning the repository, run `composer install` to install dependencies.
+
+> See <https://getcomposer.org/> for Composer installation instructions.
+
+## Testing
+
+Running the tests:
+
+```shell
+./vendor/bin/phpunit
+```
+
+See [.travis.yml](.travis.yml) for PHP versions we currently support.
+
+## Style guide
+
+We follow the [PSR-2 Style Guide](https://www.php-fig.org/psr/psr-2/), which is
+configured in [.phpcs.xml](.phpcs.xml) and can be verified by running:
+
+```shell
+./vendor/bin/phpcs
+```