Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribution Guidelines
+
+* Please Don’t edit ``README.md`` directly, edit ``data.json`` and run ``gen_readme.py`` instead (you need to have ``Jinja2`` installed). If you are not familiar with Python, open an issue to make suggestions.
+* End all descriptions with a full stop/period.
+* Add category or modify existing categories if needed.
+* Please test the extensions by yourself before making suggestions.
+* Please open an individual Pull Request for each suggestion.
+
+Thank you for your contributions!