Added contributing guidelines

CONTRIBUTING.md

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+# Contributing to Ueli
+
+First off, thanks for taking the time to contribute!
+
+This project adheres to the Contributor Covenant [code of conduct](CODE_OF_CONDUCT.md). By participating, you are expected to uphold this code. Please report unacceptable behavior to [email protected].
+
+The following is a set of guidelines for contributing to Ueli. These are just guidelines, not rules, use your best judgment and feel free to propose changes to this document in a pull request.
+
+## Issues
+
+Issues are created [here](https://github.com/oliverschwendener/ueli/issues/new).
+
+### Languages
+
+We accept issues only in English. When an issue is posted in a language besides English, it will be closed.
+
+## Pull Requests
+
+Pull requests are created [here](https://github.com/oliverschwendener/ueli/pulls)
+
+Pull Requests are the way concrete changes are made to the code, documentation, dependencies, and tools contained in the repository.
+
+### Languages
+
+We accept pull requests only in English. When a pull request is posted in a language besides English, it will be rejected.
+
+### Coding style
+
+Please use [eslint](.eslintrc) to verify that your contribution matches our coding style. Additionally we use [prettier](.prettierrc) to auto-format the code. All pull requests that don't match the coding style won't be accepted.