Add CONTRIBUTING.md.

.github/CONTRIBUTING.md

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+### Pre-implementation
+
+Before authoring a code contribution, open an issue explaining your proposal in detail.
+Remember that code is easy to write, but a valid design is usually the crux.
+Expect for the proposal to be rejected if you do not consider 100% of use-cases and corner cases.
+
+### Code style and quality
+
+Be courteous and try to match the code style.
+I won't define the exact style here, so just look at existing code.
+
+Use C++11 style, not C++future or C.
+This means using the `std::` namespace for stdlib symbols, `std::string` instead of `char*`, etc.
+Avoid pointlessly over-engineered things like `iostream`, `std::array`, `std::unique_ptr`, etc when simpler alternatives exist.
+
+Write maintainable code that will last >4 years.
+No feature/bug is urgent enough to write dirty/temporary implementations.
+I have no need to withdraw technical debt.
+
+### Legal
+
+Because a proprietary fork of VCV Rack (i.e. *Rack for DAWs*) is planned by VCV, in order for your patch to be accepted, you must make a declaration stating that:
+- you are the sole author of your patch.
+- your patch is released under the [CC0](https://creativecommons.org/publicdomain/zero/1.0/) license.
+- or ask for a copyright reassignment form if you prefer this instead.