Add contributing guidelines

CONTRIBUTING.md

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+First, thank you for taking the time to contribute! 
+
+# How to contribute
+
+## Reporting Bugs
+
+Bugs are tracked as [GitHub issues](https://github.com/codeclimate/velocity-deploy-action/issues). After you've confirmed the issue hasn't be reported before, you can open an issue explaining the problem and including additional details to help us reproduce the problem:
+
+* Use a clear and descriptive title.
+* Describe the exact steps which reproduce the problem in as much detail as possible.
+* Describe the behavior you observed after following the steps.
+* Explain which behavior you expected to see instead and why.
+* Include screenshots or animated GIFs whenever possible.
+
+## Submitting changes
+
+Please open a GitHub Pull Request with a clear title and desciption (read more about [pull requests](http://help.github.com/pull-requests/)). Please make sure all of your commits are atomic (one feature per commit). Always write a clear log message for your commits. One-line messages are fine for small changes, but bigger changes should include more details.
+
+Thank you! 
+
+Code Climate Team