make contributing guideline

CONTRIBUTING.md

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+# Contributing to flowerMD
+
+Thank you for considering contributing to flowerMD! We welcome all contributions 
+in the form of bug reports, feature requests, code contributions, suggestions, etc.
+
+The default branch for this repository is `main`. Please submit all pull requests 
+to the `main` branch.
+
+## Bug reports and feature requests
+For bug reports and feature requests, please open an issue on the
+[GitHub issue tracker](https://github.com/cmelab/flowerMD/issues). 
+
+If you are reporting a bug, please include the following information:
+- A short description of the bug
+- A minimal working example that reproduces the bug
+- The expected behavior
+- The actual behavior
+- The version of flowerMD you are using
+
+For feature requests, please include a short description of the feature you would
+like to see added to flowerMD.
+
+## Code contributions
+We welcome all code contributions in the form of pull requests. If you are
+interested in contributing code, please follow the steps below:
+- Fork the repository (default branch is `main`)
+- Create a new branch for your feature/bug fix
+- Create the `flowermd` conda environment using the `environment-cpu.yml` or `environment-gpu.yml` file
+- Make your changes
+- Install the latest version of pre-commit using `conda install -c conda-forge pre-commit` and run `pre-commit install` in the root directory of the repository 
+- Commit your changes and make sure all the pre-commit hooks pass
+- Push your changes to your fork and create a pull request
+- Assign the pull request to a reviewer and wait for the reviewers feedback
+
+New functionalities should be covered by unit tests. If you are adding a new
+function/feature/class please follow the existing style for docstrings.