feat(contrib): add Contributor-Guide

Contributor-Guide.md

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+# Contributor Guide
+
+## Code is open source, not open contribution
+
+[Similar to SQLite](https://www.sqlite.org/copyright.html), TfStated's code is
+open source but not open contribution for multiple reasons:
+
+- It avoids potential intellectual property and licensing issues.
+- It removes the burden of reviewing patches and maintaining the resulting code.
+- It helps keep the software focused on a clear vision.
+
+While this might be disappointing to you, this choice helps me continue to build
+and maintain TfStated.
+
+## Bug reports
+
+I am thankful for any bug report. Feel free to open issues and include as much
+useful information as possible. Please try to search through open issues first
+to avoid duplicates!
+
+I cannot guarantee that I will fix every bug.
+
+## Ideas and feature suggestions
+
+Ideas about existing features and suggestions for new ones are welcome, either
+on by opening issues on this forge or by
+[email](mailto:julien.dessaux@adyxax.org). I am also reachable on [mastodon
+@adyxax@adyxax.org](https://fedi.adyxax.org/@adyxax).
+
+You can also [hire me](mailto:julien.dessaux@adyxax.org) for support or to
+develop specific features.