Create CONTRIBUTING.md

Author: lucadegasperi

CONTRIBUTING.md

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+Thanks for contributing to this project.
+
+Everytime you open an issue please state which version of Laravel are you using and which version of this package are you using.
+This is very important so that problems can be issued quicker.
+
+
+Please ensure that you run `phpunit` from the project root after you've made any changes.
+
+If you've added something new please create a new unit test, if you've changed something please update any unit tests as appropritate.
+
+We're trying to ensure there is **100%** test code coverage (including testing PHP errors and exceptions) so please ensure any new/updated tests cover all of your changes.
+
+Thank you,
+
+@lucadegasperi