Update CONTRIBUTING.md

[ghost]

Updating CONTRIBUTING.md to reduce barrier to entry for contributing

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This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.

Thank you for contributing to the IoT extension!

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• If introducing new functionality or modified behavior, are they backed by unit and integration tests?
• In the same context as above are command names and their parameter definitions accurate? Do help docs have sufficient content?
• Have all unit and integration tests passed locally? i.e. pytest -s <project root>/azext_iot/tests
• Have static checks passed using the .pylintrc and .flake8 rules? Look at the CI scripts for example usage.