add contributing guide

CONTRIBUTING.md

+If you follow these contributing guidelines your patch
+will likely make it into a release a little more quickly.
+
+## Contributing
+
+1. Fork the repo.
+
+2. Create a separate branch for your change.
+
+3. We only take pull requests with passing tests, and documentation. You can also execute them locally. 
+
+4. Checkout [Voxpupuli review docs](https://voxpupuli.org/docs/#reviewing-a-module-pr). We try to 
+   use it to review a merge request and the [official styleguide](https://puppet.com/docs/puppet/6.0/style_guide.html).
+   They provide some guidance for new code that might help you before you submit a merge request.
+
+5. Add a test for your change. Only refactoring and documentation
+   changes require no new tests. If you are adding functionality
+   or fixing a bug, please add a test.
+
+6. Squash your commits down into logical components. Make sure to rebase
+   against our current master.
+
+7. Push the branch to your fork and submit a merge request.
+
+Please be prepared to repeat some of these steps as your code is reviewed.