Document CI checks that can be run locally in `CONTRIBUTING.md`

This repository's CI checks perform extensive linting, documentation checks, and testing, much of which can be done locally. It can be tricky to remember what of these must be done before opening the PR. We can document these steps in [`CONTRIBUTING.md`](https://github.com/bloomberg/blazingmq-sdk-python/blob/d3e3f4302f9bb20429ff410ec1925293c06aa873/CONTRIBUTING.md), which also discusses the DCO checks.