Add community-maintained GitHub Action for GitHub Actions workflows to README

[DevTwilight]

Hi!

I created setup-gdscript-formatter, a GitHub Action to simplify installing GDScript Formatter in GitHub Actions workflows.

It was originally built for my personal use, but after using it for a while I thought it could be useful to other people in the community as well.

I wanted to share it here in case it is useful for other GDScript Formatter users.

Thanks!

[NathanLovato]

Thanks! Let's add it to the README.

[DevTwilight]

ok will create a pr

[NathanLovato]

Closed by #290