Add platform-specific setup instructions or tips to workflow.md

[mattvlaw]

I have had different experiences getting the site running locally on different platforms. Would eventually like to curate walkthroughs for major platforms (MacOS, Windows, Ubuntu 18/19) to document these for others (e.g. install jekyll through bundler not apt on Ubuntu 18.04).

[mattvlaw]

Also link to workflow.md from the readme.md