Add the contributor guide

CONTRIBUTING.md

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+# Contributing to MetaWear-API-Swift
+
+Thanks for your interest in contributing! This document explains how to set up
+the project, the workflow we follow, and the conventions we expect in pull
+requests.
+
+By contributing, you agree that your contributions are licensed under the terms
+in [`LICENSE.md`](LICENSE.md).
+
+## Code of conduct
+
+Be respectful and constructive. Harassment or abuse of any kind is not
+tolerated. Report concerns to [hello@mbientlab.com](mailto:hello@mbientlab.com).
+
+## Ways to contribute
+
+- **Report a bug** — open an issue with steps to reproduce, the board model
+  (MetaMotion S / RL) and firmware revision, the SDK version, and what you
+  expected vs. what happened.
+- **Request a feature** — open an issue describing the use case before sending a
+  large PR, so we can agree on the approach first.
+- **Send a pull request** — small, focused PRs are easiest to review. For
+  anything substantial, open an issue first.
+
+### Reporting a security issue
+
+**Do not open a public issue for security vulnerabilities.** Use GitHub's private
+vulnerability reporting (the **Security** tab → *Report a vulnerability*) or
+email [hello@mbientlab.com](mailto:hello@mbientlab.com).
+
+## Development setup
+
+Requirements:
+
+- macOS with a recent **Xcode** (matching the project's Swift toolchain)
+- **Swift 6** (the package builds with strict concurrency enabled)
+- A physical **MetaMotion S** or **MetaMotion RL** board to run the hardware test
+  suite (not required for the unit tests)
+
+Clone and build:
+
+```bash
+git clone git@github.com:mbientlab/MetaWear-API-Swift.git
+cd MetaWear-API-Swift
+swift build --build-tests
+```
+
+## Building and testing
+
+The unit suite runs entirely against a mock transport — no hardware or
+Bluetooth needed. **Run it before every PR:**
+
+```bash
+swift test --filter MetaWearTests
+```
+
+The **hardware suite** (`MetaWearHardwareTests`) talks to a real board over
+Bluetooth and is intended to be run manually from Xcode with a board powered on
+and in range. It is **not** run in CI.
+
+CI runs `swift build --build-tests` and `swift test --filter MetaWearTests` on
+every pull request via [`.github/workflows/ci.yml`](.github/workflows/ci.yml).
+**Your PR must be green before it can merge.**
+
+## Pull request workflow
+
+1. Create a topic branch off `main` (e.g. `your-name/short-description`).
+   `main` is protected — don't push to it directly.
+2. Make your change. Add or update tests for any behavior change.
+3. Run `swift test --filter MetaWearTests` locally.
+4. Open a PR against `main` with a clear description of *what* and *why*.
+5. Make sure the `build-and-test` check passes and address review feedback.
+
+Keep each PR scoped to a single concern. Unrelated changes (formatting sweeps,
+renames) belong in their own PR.
+
+## Commit messages
+
+- Write a concise summary line in the imperative mood
+  (e.g. "Fix logger cleanup on download timeout").
+- Add a body explaining the reasoning when the change isn't self-evident.
+- Reference issues where relevant (e.g. `Fixes #123`).
+
+## Code style
+
+- Follow the
+  [Swift API Design Guidelines](https://www.swift.org/documentation/api-design-guidelines/).
+- Match the style, naming, and comment density of the surrounding code.
+- Prefer Swift concurrency (`actor`, `async`/`await`, `AsyncThrowingStream`) over
+  GCD or Combine, consistent with the existing architecture.
+- **The library must never crash a host app on bad input.** Validate caller
+  input with a thrown `MWError` rather than `precondition`/`fatalError`.
+- Keep one primary type per file; group files by feature.
+- Document public API with `///` doc comments, including `- Throws:` for throwing
+  initializers and methods.
+
+## Protocol changes
+
+This SDK is a clean-room implementation of the MetaWear BLE protocol. When you
+change anything that touches the wire format, cite the authoritative source for
+the byte layout (the protocol spec and/or the reference C++ SDK) in the code or
+PR, and add a byte-exact regression test under `Tests/MetaWearTests`.
+
+## Questions
+
+Open a discussion or issue, or email
+[hello@mbientlab.com](mailto:hello@mbientlab.com). Thanks for contributing!