Improve developer experience: Add contributing guidelines

### Context

The idea of "improving developer experience" has been coming up around the office consistently + the idea of contributing guidelines came up in [a discussion](https://github.com/AlexsLemonade/refinebio-docs/issues/55#issuecomment-426674400) around the introduction section of the documentation.

### Problem or idea

To my knowledge, the most information we have around contributing is the [Git Workflow section](https://github.com/AlexsLemonade/refinebio#git-workflow) of the README. There very well could be more bits and pieces in other markdown files throughout the repository, but it is not collected in one place AFAIK.

### Solution or next step

Add contributing guidelines (e.g., `CONTRIBUTING.md`)

### New Issue Checklist

- [x] The title is short and descriptive.
- [x] You have explained the context that led you to write this issue.
- [x] You have reported a problem or idea.
- [x] You have proposed a solution or next step.

Start by reviewing the Git Workflow section of the README and searching the repository's Markdown files for existing contribution information. Use the linked documentation discussion for context, then consolidate the relevant guidance in a new CONTRIBUTING.md file; done means contributors can find the project's contribution instructions in one place.

Written by the indexing model from the issue text.