Add rudimentary CONTRIBUTING.md

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.
+
+## Architecture
+
+The main components are:
+
+- **`Session` class** *(entrypoint)*
+
+    - `Session.Builder` is used to configure and create a session, via one of:
+
+        - username/password
+        - stored credentials
+        - OAuth
+
+    - An active session is **required** for all other operations.
+
+- **`ApiClient` class**
+
+    - A high-level client for making standard HTTPS requests to Spotify's Web API endpoints (e.g., `https://spclient.wg.spotify.com`).
+    - Accessed via `session.api()`, it provides convenient methods like `get_metadata_4_track()` and handles client tokens automatically.
+
+- **`MercuryClient` class**
+
+    - The low-level client for Spotify's proprietary `mercury` protocol, which uses `hm://` URIs.
+    - Accessed via `session.mercury()`, it handles sending and receiving messages over the main session connection for metadata lookups and subscriptions that are not available via the standard Web API.
+
+- **`DealerClient` class**
+
+    - Manages the persistent WebSocket (`wss://`) connection to Spotify's `dealer` service.
+    - Accessed via `session.dealer()`, it listens for and dispatches real-time, asynchronous JSON-based events, such as remote player state changes or notifications from other connected devices.
+
+- **`Session.Receiver` thread**
+
+    - Spawned after authentication to read every encrypted packet coming from the access point.
+    - Routes decoded commands to subsystems (`MercuryClient`, `AudioKeyManager`, `ChannelManager`, etc.) and responds to keep-alive pings to hold the session open.
+
+- **Metadata types**
+
+    - The `librespot.metadata` module provides typed identifiers (`TrackId`, `AlbumId`, `PlaylistId`, `EpisodeId`, etc.) used to reference Spotify content throughout the API.
+    - They are constructed from Spotify identifiers, typically using one of the following methods:
+
+        - `from_uri()`: For all ID types.
+        - `from_base62()`: For most ID types (e.g., tracks, albums, artists).
+
+- **`PlayableContentFeeder` class**
+
+    - Retrieves audio streams; is accessed via `session.content_feeder()`.
+    - `load(playable_id, audio_quality_picker, preload, halt_listener)`:
+
+        - Accepts:
+
+            - a `TrackId` or `EpisodeId` (any `PlayableId`)
+            - an `AudioQualityPicker`
+            - a `preload` flag
+            - an optional `HaltListener` callback (pass `None` if unneeded).
+
+        - Returns a `LoadedStream` that contains the decrypted stream together with:
+
+            - track/episode metadata
+            - normalization data
+            - transfer metrics
+
+- **`audio` module**
+
+    - Contains tools for format selection, quality management, streaming, and decryption.
+    - `VorbisOnlyAudioQuality` and `LosslessOnlyAudioQuality` choose the best matching `Metadata.AudioFile` for a preferred container/quality combination.
+    - `CdnManager` acquires and refreshes signed CDN URLs, feeding a `Streamer` that decrypts chunks on the fly while staying seekable.
+
+- **`AudioKeyManager` and `ChannelManager`**
+
+    - Handle the low-level transport for protected audio: `AudioKeyManager` requests AES keys, and `ChannelManager` can stream encrypted chunks directly from the access point when CDN delivery is unavailable.
+    - Both are driven transparently by `PlayableContentFeeder`/`CdnManager`, so callers only interact with the decrypted `LoadedStream`.
+
+- **`EventService` class**
+
+    - Asynchronous publisher that emits telemetry (e.g., fetch metrics, playback events) to `hm://event-service/v1/events` via Mercury.
+    - Accessible through `session.event_service()` for consumers that need to forward custom events.
+
+- **`TokenProvider` class**
+
+    - Caches Login5 access tokens per scope, refreshing them proactively as they near expiry.
+    - Used by `ApiClient` to supply the correct `Authorization` headers for Spotify Web API calls.
+
+- **`SearchManager` class**
+
+    - High-level wrapper around `hm://searchview/km/v4/search/...` requests sent over Mercury.
+    - Fills in username, locale, and country defaults from the current session before dispatching the call.
+
+- **OAuth tokens for Spotify Web API**
+
+    - Can be obtained via `session.tokens().get(scope)`
+    - Enable authenticated API calls for operations like search, playlist management, and user data access