feat(repo): add CONTRIBUTING file

CONTRIBUTING.md

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+# Coding Conventions
+
+* Use 4 spaces for indentation.
+* Code conforms to PEP8. Use pep8 or pycodestyle with their default settings to check for compliance. Consider using a pre-commit hook to prevent non-conforming code from entering the repo.
+* Avoid inline comments, prefer (short) block comments.
+* Add documentation strings to function if required.
+* If you want, add yourself to the AUTHORS file.
+
+# Commit Conventions
+
+* Follow [conventional commit](https://www.conventionalcommits.org) message guidelines.
+* Scopes for commit messages are `etc`, `src` and file names from the root directory. Take a look at git log to get an impression.
+* You can ignore the 50 chars limit for the first line of a commit message and obey to a hard limit of 72 chars.