Initial contributing guidelines

CONTRIBUTING.md

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+# Contributing to pygfunction
+
+*pygfunction* welcomes and appreciates bug reports, suggestions and
+contributions from everyone.
+
+This guide describes how to report bugs, suggest new features and
+contribute code to *pygfunction*.
+
+
+## Reporting bugs
+
+Bugs are reported on the
+[issue tracker](https://github.com/MassimoCimmino/pygfunction/issues).
+
+Follow these steps when submitting a bug report:
+
+1. **Make sure the bug has not been already reported.** Run a quick search
+through the [issue tracker](https://github.com/MassimoCimmino/pygfunction/issues).
+If an open issue is related to your problem, consider adding your input to that
+issue. If you find a closed issue related to your problem, open an new issue
+and link to the closed issue.
+2. **Use a short and descriptive title** when creating a new issue.
+3. **Provide detailed steps to reproduce the problem.** Explain, in details,
+how to reproduce the problem and what should be the **expected result.** When
+possible, include a simple code snippet that isolates and reproduces the
+problem.
+
+After submitting a bug report, if you wish to contribute code to fix the
+problem, follow the steps outlined in the contribution workflow.
+
+
+## Contribution workflow
+
+This section outlines the steps for contributing to *pygfunction*.
+
+1. **Open a new [issue](https://github.com/MassimoCimmino/pygfunction/issues).**
+2. **Use a short and descriptive title.** When proposing an enhancement,
+describe in details what the enhancement would entail. If you plan to implement
+the enhancement yourself, provide a step-by-step plan for the implementation.
+3. **Explain how the enhancement benefits _pygfunction_.**
+4. **Create (checkout) a new branch from the master.** The branch name should
+follow the naming convention: `issue#_shortDescription`. For example:
+issue1_unitTests.
+5. Before submitting a
+[pull request](https://github.com/MassimoCimmino/pygfunction/pulls), **merge
+the master to your branch.**
+6. Once the branch is merged, **delete the branch and close the issue.**
+
+
+## Styleguide
+
+*pygfunction* follows the
+[PEP8 style guide](https://www.python.org/dev/peps/pep-0008).
+Docstrings are written following the
+[numpydoc format](https://github.com/numpy/numpy/blob/master/doc/example.py),
+see also an example
+[here](https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_numpy.html).