Docs: Add code of conduct and contributing docs

Author: pedroamador

CODE_OF_CONDUCT.md

@@ -0,0 +1,4 @@
+This project is governed by the Contributor Covenant version 1.4
+(https://www.contributor-covenant.org/version/1/4/code-of-conduct.html).  
+All contributors and participants agree to abide by its terms. 
+To report violations, send an email to pedroamador [DOT] rodriguez [AT] gmail [DOT] com

CONTRIBUTING.md

@@ -0,0 +1,52 @@
+# Contributing guide
+
+We love your input! We want to make contributing to this project as easy and transparent as possible, whether it's:
+
+- Reporting a bug
+- Discussing the current state of the code
+- Submitting a fix
+- Proposing new features
+- Becoming a maintainer
+
+## We Develop with Github
+
+We use github to host code, to track issues and feature requests, as well as accept pull requests.
+
+## We Use [Github Flow](https://guides.github.com/introduction/flow/index.html), So All Code Changes Happen Through Pull Requests
+
+Pull requests are the best way to propose changes to the codebase (we use [Github Flow](https://guides.github.com/introduction/flow/index.html)). We actively welcome your pull requests:
+
+1. Fork the repo and create your branch from `master`.
+2. If you've added code that should be tested, add tests.
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Make sure your code lints.
+6. Issue that pull request!
+
+## Any contributions you make will be under the MIT Software License
+
+In short, when you submit code changes, your submissions are understood to be under the same [MIT License](http://choosealicense.com/licenses/mit/) that covers the project. Feel free to contact the maintainers if that's a concern.
+
+## Report bugs using Github's issues
+
+We use GitHub issues to track public bugs. Report a bug by [opening a new issue](https://github.com/tpbtools/gp-php/issues/new); it's that easy!
+
+## Write bug reports with detail, background, and sample code
+
+**Great Bug Reports** tend to have:
+
+- A quick summary and/or background
+- Steps to reproduce
+  - Be specific!
+  - Give sample code if you can
+- What you expected would happen
+- What actually happens
+- Notes (possibly including why you think this might be happening, or stuff you tried that didn't work)
+
+## License
+
+By contributing, you agree that your contributions will be licensed under its MIT License.
+
+## References
+
+Based on https://gist.github.com/briandk/3d2e8b3ec8daf5a27a62 adapted from the open-source contribution guidelines for [Facebook's Draft](https://github.com/facebook/draft-js/blob/a9316a723f9e918afde44dea68b5f9f39b7d9b00/CONTRIBUTING.md)