Local development docs

Hey there, 

love the project! However, Im missing a couple of things: 
- Local development setup: I'm not a Ruby dev, so I dont know how can I deploy the project locally to test out the changes before pushing them to GH pages. Ideally that would be on host machine or using Docker (even better!)
- When the above is solved, I would add an action to build and/or preview the page on PR.