Create CONTRIBUTING.md

Author: adam-fowler

CONTRIBUTING.md

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+# Contributing
+
+## Legal
+By submitting a pull request, you represent that you have the right to license your contribution to the community, and agree by submitting the patch
+that your contributions are licensed under the Apache 2.0 license (see [LICENSE](LICENSE.txt)).
+
+## Contributor Conduct
+All contributors are expected to adhere to the project's [Code of Conduct](CODE_OF_CONDUCT.md).
+
+## Submitting a bug or issue
+Please ensure to include the following in your bug report
+- A concise description of the issue, what happened and what you expected.
+- Simple reproduction steps
+- Version of the library you are using
+- Contextual information (Swift version, OS etc)
+
+## Submitting a Pull Request
+
+Please ensure to include the following in your Pull Request
+- A description of what you are trying to do. What the PR provides to the library, additional functionality, fixing a bug etc
+- A description of the code changes
+- Documentation on how these changes are being tested
+- Additional tests to show your code working and to ensure future changes don't break your code.
+
+Please keep your PRs to a minimal number of changes. If a PR is large try to split it up into smaller PRs. Don't move code around unnecessarily it makes comparing old with new very hard.
+
+The main development branch of the repository is  `main`.
+
+### Testing
+
+The project tests expect a local version of mosquitto to be running using the config that is in this project. You can start a local mosquitto MQTT server by running the script `scripts/mosquitto.sh`. You can test the Linux version using Docker. There is a docker-compose file in the root of this project. Run Linux tests as follows
+```
+docker-compose run test
+```
+
+### Formatting
+
+We use Nick Lockwood's SwiftFormat for formatting code. PRs will not be accepted if they haven't be formatted. The current version of SwiftFormat we are using is v0.48.17.