Update Getting Started section of README.md

[dgcole]

Not sure if there is any procedure for PRs on this repo / template that I am missing, hopefully not.

When updating, I noticed that the implementation of the demo extension was changed with #1095 , but the corresponding README.md sections were not updated, so I figured I might as well make a PR.

[dsnopek]

Ah, thanks for catching that! This looks good to me :-)

[paddy-exe]

Looks good to me as well! Nice catch!

[mhilbrunner]

This should update the note at the top of the README to say that master is 4.1+ and to include 4.0 in the branches mentioned people may want to check out. Otherwise looks good to me.

[dgcole]

This should update the note at the top of the README to say that master is 4.1+ and to include 4.0 in the branches mentioned people may want to check out. Otherwise looks good to me.

Good call, I've updated the warning to clarify that the current version on master is for 4.1+, and I added a note to switch to 4.0 for 4.0.x versions of the engine.

[paddy-exe]

@dgcole This needs a squash to be merged.

[dsnopek]

This document is helpful for squashing your commits into a single one:

https://docs.godotengine.org/en/stable/contributing/workflow/pr_workflow.html#the-interactive-rebase

There also appears to be a "coding standards" failure in CI: https://github.com/godotengine/godot-cpp/actions/runs/5060252047/jobs/9087748389?pr=1115

It looks like a trailing space at the end of this line:

> This repository's `master` branch is only usable with the latest version of 

[dsnopek]

Thanks!