Add first draft of CONTRIBUTING.md

Author: akbad

CONTRIBUTING.md

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+# Contributing
+
+## What this library is
+
+- A headless Spotify client, allowing you to **authenticate and retrieve a decrypted audio stream for any track**.
+- *Not* a standalone audio player: the **provided stream must be piped to another application** (like `ffplay`) or handled by a server to be played.
+
+## Environment setup
+
+### Prerequisites
+- Python 3.10+
+
+### Install runtime packages
+
+```sh
+pip install -r requirements.txt
+```
+
+### Install protoc
+
+> This step is **only needed if you're changing any `.proto` serialization schema files**,
+> which will subsequently require using the protoc compiler to generate updated versions of 
+> the `*_pb2.py` Python stubs that implement serialization/deserialization for those schemas.
+
+- Go to the [protobuf release matching the version pinned in `requirements.txt`](https://github.com/protocolbuffers/protobuf/releases/tag/v3.20.1).
+- Download and install the `protoc-*.zip` file meant for your platform.
+
+After modifying the `.proto` files you need to, **make sure to follow [these steps](#protocol-buffer-generation) to regenerate the Python stubs**.
+
+## Protocol buffer generation
+
+> These steps are only necessary after changing `.proto` files.
+
+- From the repository root, conveniently recompile all `.proto` schema files with this command:
+
+    ```bash
+    find proto -name "*.proto" | xargs protoc -I=proto --python_out=librespot/proto
+    ```
+
+- Alternatively, to recompile a single file (e.g. `proto/metadata.proto`), run:
+
+    ```bash
+    protoc -I=proto --python_out=librespot/proto proto/metadata.proto
+    ```
+
+- Commit both the source `.proto` and the regenerated Python output **together** so they can
+be compared easily.