Add contribution guidelines

CONTRIBUTING.md

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+# Contributing to Abby
+
+I 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
+
+## Pull requests are actively welcomed
+
+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.
+5. Make sure your code lints.
+6. Issue your 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.
+
+## Report bugs using [issues](https://github.com/andreekeberg/abby/issues)
+
+All bugs are tracked using GitHub issues to track public bugs. Report a bug by [opening a new issue](https://github.com/andreekeberg/abby/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)
+
+## Use a Consistent Coding Style
+
+I'm using the automatic formatter in the [PHP Intelephense](https://marketplace.visualstudio.com/items?itemName=bmewburn.vscode-intelephense-client) Visual Studio Code extension. 
+
+* 4 spaces for indentation rather than tabs
+* Newlines after opening curly brackets in classes and methods
+
+## License
+
+By contributing, you agree that your contributions will be licensed under its MIT License.