Add contribution guide

CONTRIBUTING.md

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+# Contributing to ProbeQuest
+
+ProbeQuest is a free and open-source software, which means that you can
+contribute to its improvement. The following document is a set of guidelines to
+help you in this process. Thank you for taking the time to read it!
+
+## Types of Contributions
+
+### Issues
+
+Several templates are available when [creating a new issue][new issue]:
+
+* Bug report
+* Feature request
+* Report a security vulnerability
+
+Please select the appropriate one and follow the instructions. Also, make sure
+to:
+
+* Check for duplicates before creating your issue
+* Choose a descriptive title
+
+If you want to ask a question, please open a new [discussion][discussions]
+instead of a new issue.
+
+### Pull Requests
+
+ProbeQuest follows the [gitflow][gitflow] branching model, which means that your
+pull request needs to target the `develop` branch.
+
+When introducing new code to ProbeQuest, please make sure to add the appropriate
+tests and documentation that cover your changes.
+
+## Development Environment
+
+To set up your development environment, please read the ["Development"
+section][development] of ProbeQuest's documentation.
+
+ [development]: https://probequest.readthedocs.io/en/latest/development.html "Development - ProbeQuest's documentation"
+ [discussions]: https://github.com/SkypLabs/probequest/discussions "GitHub Discussions"
+ [gitflow]: https://nvie.com/posts/a-successful-git-branching-model/ "A successful Git branching model"
+ [new issue]: https://github.com/SkypLabs/probequest/issues/new/choose "Create a new issue"