Add contributing guide.

CONTRIBUTING.md

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+# How to contribute
+
+Thank you for considering contributing to rdata! You can contribute to rdata in several ways.
+
+## Opening issues
+
+Open a new issue if you find some bug or you want to propose a new feature. Please first check that nobody has asked that already!
+
+Make sure that your proposal is clearly written, preferably in English. In case you are reporting a bug, please include all relevant information, such as the software version and machine information.
+
+## Discussing the project
+
+You can open a discussion for any topic related with this package. Do you have doubts about how to use the package? Open a discussion! Do you want to show related projects, recent research or some use case for this software? Open a discussion!
+
+You are also encouraged to answer the discussions of other users and participate actively in the discussions forum.
+
+## Improving the documentation
+
+Do you feel that the documentation could be more clear? Did you found a typo? You can easily edit the documentation and make a pull request clicking the "Edit this page" link in the documentation.
+
+Advanced users can also propose the addition of new pages and examples. In case you want to do that, please open an issue to discuss that first.
+
+## Contributing software
+
+You can improve this package by adding new functionality, solving pending bugs or implementing accepted feature requests. Please discuss that first to ensure that it will be accepted and to assign that to you and prevent duplicated efforts.
+
+In any case, make sure that you own the rights to the software and are ok with releasing it under a MIT license.