doc: add contributing.md

CONTRIBUTING.md

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+# Contributing
+
+If you're considering contributing, please open an issue before a PR. A lot of discussion
+also happens in the [Neorg Discord](https://discord.gg/T6EgTAX7ht), so you might consider
+joining.
+
+## Tests
+
+If you change a behavior or fix a bug, please make sure to add a test for it!
+
+- run the test suite with `cargo test`
+
+There are snapshot tests and prop tests. If you change the parser behavior or add a new
+test case, the snapshots will change and you will see a test failure. You can approve the
+new version of the snapshot with:
+
+- `cargo insta review`
+
+Prop tests essentially fuzz the parser and make sure that it doesn't panic. Failed test
+cases are saved and version controlled to avoid regressions.
+
+<!-- vim: set tw=85 -->