Add instructions to setup local environment

[rickychilcott]

Documentation

Description

Adds some simple instructions to get a local development environment working. This could be helpful to people who are used to working on gems but not necessarily npm packages and how yarn works.

Checklist

• My code follows the style guidelines of this project
• Checks (StandardRB & Prettier-Standard) are passing

[rickychilcott]

BTW @julianrubisch, these standardrb checks are failing because a8ed557 broke the styling. I fixed it in d2509b3. Do mind cherry-picking that commit into master and then I can merge master into other branches?

[julianrubisch]

Yeah I believe the standardrb version was bumped because prior to that it did work. Nevermind for now, I‘ll cherry pick that later!