Add CONTRIBUTING.md

CONTRIBUTING.md

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+## How to contribute
+
+First of all thank you for considering it. Check out
+[ARCHITECTURE.md](ARCHITECTURE.md) for an overview of how the repo is put together.
+
+### Running tests
+
+You won't be able to run the tests without enabling the `macros` feature
+as one of the examples depend on it.
+
+`cargo test --feature macros`
+
+
+### Debugging the proc macros
+
+To see the output of the macros enable the `dump-macro` feature.
+For the macro example the output can be viewed like this:
+
+```sh
+cargo r --example macro --features dump-macro,macros -- dark | rustfmt --edition=2021 | less
+```