add CONTRIBUTING

CONTRIBUTING.md

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+# Contributing
+## Introduction
+
+First off, thank you for considering contributing to TextBrewer.
+
+TextBrewer is an open source project and we love to receive contributions from our community. There are many ways to contribute,  including but are not limited to:
+
+* Writing tutorials and adding examples
+* Improving the documentation
+* Submitting new features, such as new new distillation methods
+* Reporting bugs
+
+The following is the guidelines for contributing to TextBrewer. Also, feel free to propose changes to this document.
+
+Everyone participating in the project should follow the [Code of Conduct](CODE_OF_CONDUCT.md).
+
+## Before Making a Change
+
+* If you have found any bugs or have any features that you wish to added, please first discuss via the issue.
+
+## Pull Request Process
+
+1. Create your own fork of the code and clone it to your local computer
+2. In your local repository, checkout a new branch and commit your changes
+3. Push your branch back to the fork on GitHub
+4. Submit a pull request from your fork, and describe your changes
+
+
+## Coding conventions
+
+* Indent with four spaces
+* Naming conventions: `lower_case_with_underscores` for variables, methods and functions; `CapWords` for class names
+* Remember to add or update the docstrings and/or README.md after making any changes to the classes, methods or functions
+