Improve contributor experience

[samber]

Hi there,

I'm adding some documentation for helping contributors get into the project.

Also adding an improvement to the "repl" script:

• when running the script in a loop, it defaults to the same URL instead of https://react-redux.realworld.io
• just type <enter> to fall back on the default URL instead of copy-pasting https://react-redux.realworld.io into the prompt

[YunFeng0817]

Thank you so much.