Add Gotcha hint to README #39

[julianrubisch]

Documentation

Description

Add a Gotcha section with a hint on how to fix the ActiveStorage URL error in development.

Fixes #30

Checklist

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

[leastbad]

Important note: production.rb needs to be set, too!

config.action_controller.default_url_options = {host: "mysite.com"}