Add instructions for custom 404.html page

[OwenFlood]

It might be helpful to add some information for people looking to get around the routing issues using the 404 strategy. It took me a lot of troubleshooting to realize that to have an HTML file passed through the build it needs to be in the public folder.

Adding some instructions could save many people time 😄.

[stixbunny]

Thank you! Was just trying to figure this out.