added contributing file

CONTRIBUTING.md

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+# Contributing to PhyloNetworks
+
+The following guidelines are designed for contributors of `PhyloNetworks`.
+
+## Reporting Issues and Questions
+
+For reporting a bug, a failed function or requesting a new feature, you can simply open an issue in [the issue tracker](https://github.com/crsl4/PhyloNetworks.jl/issues).
+First, seach through existing issues (open or closed) that might have the answer to your question.
+If you are reporting a bug, please also include a minimal code example or all relevant information for us to replicate the issue.
+
+For general questions, make sure to check out the [google user group](https://groups.google.com/g/phylonetworks-users). If you cannot find answers to your question, please post a new question. We do our best to reply in a timely fashion, but we are undermanned so we appreciate your patience.
+
+## Contributing Code
+
+To make contributions to `PhyloNetworks`, you need to set up your [GitHub](https://github.com/) account (if you do not have one) and request your change(s) or contribution(s) via a pull request against the `master` branch of `PhyloNetworks` from a non-master branch in your fork.
+Using a non-master branch on your end will give developers push access to your
+branch to make edits to it (in case we want to work collaboratively on the new code).
+
+Please use the following steps:
+
+1. Fork the `PhyloNetworks` repository to your GitHub account
+2. Clone your fork locally with `git clone`
+3. Create a new branch with a name that describes your contribution. For example, if your contribution is fixing a bug in `readTopology`, your new branch can be named `fix-bug-readTopology`. You can create it and switch with:
+   ```
+   git checkout -b fix-bug-readTopology
+   ```
+4. Make your changes on this new branch.
+   Write a [docstring](https://docs.julialang.org/en/v1/manual/documentation/)
+   and add one or more tests for each function.
+   Make sure that your code passes all the
+   [tests](https://pkgdocs.julialang.org/v1/managing-packages/#Testing-packages).
+5. Push your changes to your fork
+6. [Submit a pull request](https://github.com/crsl4/PhyloNetworks.jl/pulls) against the `master` branch in `PhyloNetworks`. Make sure that your code passes all the automatic tests and that it is not in conflict with the current status of `master`
+
+Please make sure to follow the Julia package guidelines and conventions on your code. `PhyloNetworks` was created before these conventions were catalyzed, but we are attempting to follow them going forward. 
+
+To learn more about the Julia conventions, check out the following links:
+
+- [Julia package development](http://web.mit.edu/julia_v0.6.0/julia/share/doc/julia/html/en/manual/packages.html#Package-Development-1)
+- [Creating Julia packages](https://pkgdocs.julialang.org/v1/creating-packages/)
+- [Julia style guide](https://docs.julialang.org/en/v1/manual/style-guide/)