Adding CONTRIBUTING.md file (#97)

* Adding CONTRIBUTING.md file

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: nathaliaesper

CONTRIBUTING.md

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+# ActiSleep Tracker pull request guidelines
+
+Pull requests are always welcome, and we appreciate any help you give. Note that a code of conduct applies to all spaces managed by the ActiSleep Tracker project, including issues and pull requests. Please see the [Code of Conduct](CODE_OF_CONDUCT.md) for details.
+
+When submitting a pull request, we ask you to check the following:
+
+1. **Unit tests**, **documentation**, and **code style** are in order.
+   See the Continuous Integration for up-to-date information on the current code style, tests, and any other requirements.
+
+   See the following [Google Python style guide](https://google.github.io/styleguide/pyguide.html#3164-guidelines-derived-from-guidos-recommendations) for coding style guidelines.
+
+   It is also OK to submit work in progress if you're unsure of what this exactly means, in which case you'll likely be asked to make some further changes.
+
+2. The contributed code will be **licensed under the same [license](LICENSE) as the rest of the repository**, If you did not write the code yourself, you must ensure the existing license is compatible and include the license information in the contributed files, or obtain permission from the original author to relicense the contributed code.