Add ARCHITECTURE, CODE_OF_CONDUCT, CONTRIBUTING files + flesh out the README.

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+## Repository architecture
+
+The architecture of this repository is a bit unusual because we are using some tricks to have both
+the 2D and 3D version of Rapier share the same code-base. Here are the main folders:
+- **`build/`**: contains one folder per Rapier crate (for the 2D, 3D, `f32`, and `f64` versions). Each
+  crate has its own `Cargo.toml` file that adjusts some cargo features, and reference the `src` folder.
+- **`src/`**: contains the actual `.rs` source code of the Rapier physics engine.
+- **`src_testbed/`**: contains the `.rs` source code of the Rapier testbed (which our examples are based on).
+- **`examples2d/`**: simple 2D scenes showcasing some of Rapier's capabilities.
+  Run them with `cargo run --release --bin all_examples2`.
+- **`examples3d/`**: simple 3D scenes showcasing some of Rapier's capabilities.
+  Run them with `cargo run --release --bin all_examples3`.
+- **`benchmarks2d/`**: a set of 2D stress-tests, to see how Rapier performs when it has lots of elements
+  to simulate.
+- **`benchmarks3d/`**: a set of 3D stress-tests, to see how Rapier performs when it has lots of elements
+  to simulate. We use the these benchmarks to track the performances of Rapier after some changes,
+  and spot unexpected regressions: https://www.rapier.rs/benchmarks/
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