Create CONTRIBUTING.md

Author: eelstork

CONTRIBUTING.md

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+# Prolog - Contributing
+
+You are welcome to submit issues and pull requests; before submitting a pull request, do consider that 
+
+- *You do not retain copyright after your contributions are merged*. Submission of a pull request implies acceptance of the 
+below Copyright transfer agreement.
+- Prolog is commercial software; as such pricing and/or license terms may change without notice; these changes are not 
+retroactive.
+
+The library's main author is not currently affiliated to an organization. As such, funding is important to help 
+developing the software. This motivates adopting Fair Source and charging small fees for organizational 
+uses. 
+
+If you find this software helpful, encourage the author [via ko-fi](https://ko-fi.com/A0114I97)
+
+## Copyright transfer agreement
+
+Effective upon merging of your pull request(s), and in exchange for T.E.A de Souza ("Activ") making Prolog fairly and 
+publicly available, as well as reviewing, editing and publishing your contributions, you hereby irrevocably transfer to Activ 
+full ownership of all rights in your contributions, including copyright.