retrie-2.0.0: CONTRIBUTING.md

# Contributing to retrie
We want to make contributing to this project as easy and transparent as
possible.

## Pull Requests
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. Please add a line to the 'Unreleased' section of `CHANGELOG.md` as part of your pull request.

## Issues
We use GitHub issues to track bugs. Please ensure your description is
clear and has sufficient instructions to be able to reproduce the issue.
Ideally, add a failing test.

## Coding Style  
* 2 spaces for indentation rather than tabs
* 80 character line length
* Avoid using whitespace to vertically align things unnecessarily.

## License
By contributing to retrie, you agree that your contributions will be licensed
under the LICENSE file in the root directory of this source tree.