Add CONTRIBUTING guidelines

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contribution Guidelines
+You are welcome to report [issues](https://github.com/chadicus/dom-php/issues) or submit [pull requests](https://github.com/chadicus/dom-php/pulls).  While the below guidelines are necessary to get code merged, you can submit pull requests that do not adhere to them and I will try to take care of them in my spare time. If you can make sure the build is passing 100%, that would be very useful.
+
+I recommend including details of your particular usecase(s) with any issues or pull requests.
+
+## Questions and Bug Reports
+Submit via [GitHub Issues](https://github.com/chadicus/dom-php/issues).
+
+## Pull Requests
+Code changes should be sent through [GitHub Pull Requests](https://github.com/chadicus/dom-php/pulls).  Before submitting the pull request, make sure that phpunit reports success:
+
+```sh
+./vendor/bin/phpunit --coverage-html coverage
+```
+
+While the build does not enforce 100% [PHPUnit](http://www.phpunit.de) code coverage, it will not allow coverage to drop below its current percentage.
+
+The build will also not allow any errors for the [coding standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md).
+
+```sh
+./vendor/bin/phpcs --standard=PSR2 src tests
+```