Add contribution guide.

docs/CONTRIBUTING.md

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+# Contributing to Reynard
+
+> The following was written by me, Manfred, so contrary to the other documentation this has a slightly more personal tone 🙂.
+
+Thank you for considering spending your time on contributing to Reynard!
+
+Reynard is a personal project and tries to implement my personal vision of an web service client with a fixed schema in an external document. Some of these ideas are written down in the [notes](https://github.com/Manfred/Reynard/blob/main/docs/notes.md).
+
+I have decided that I will not accept any code in this project unless it's exactly how I would have written it myself. Meaning that usually the fastest way to get code into the repository is to let me write it.
+
+If you have a feature request, please start by taking the following steps:
+
+1. Open a Feature issue, and;
+2. Explain **why** and **what** you want added, and;
+3. Include snippets or entire OpenAPI specifications to show your feature, bonus points if you add a few variations that can be included in the automated tests directly, but;
+4. Keep communication short and to the point.
+
+Same goes for defects:
+
+1. Open a Defect issue, and;
+2. Explain what you **expect** and what **actually happened** using actual data that doesn't include personal information, but;
+3. You don't have to write down an entire code trace, I have a pretty good grasp on the code, so;
+4. Keep communication short and to the point.
+
+I have contributed to open source a lot and have experienced most difficult situations both as a maintainer and a contributor, so I know it can be hard or frustrating. You can always hire me though [Fingertips](https://fngtps.com) if you have specific needs. In all other cases you can consider maintaining a fork of the project when you are in a hurry or don't agree with my ideas.
+
+Thanks for reading!