Update README.md with basic pre-requisites

[nightblade9]

No description provided.

[YuriSizov]

Thanks for your contribution! Instead of an ad hoc suggestion in the readme, it's worth it to link to the Godot buildsystem documentation that already explains how to install scons on each supported platform:

https://docs.godotengine.org/en/stable/contributing/development/compiling/index.html

[nightblade9]

Thanks, I wasn't aware of this (I'm not a C/C++ developer). I also wasn't aware that godot-cpp has the same build requirements as the editor. I'll update my PR momentarily.

[YuriSizov]

I also wasn't aware that godot-cpp has the same build requirements as the editor.

I'm not sure it has exactly the same requirements, but as far as installing scons, engine's docs are already very comprehensive and should be more suitable than a short mention like your original edit was.

[dsnopek]

Thanks!

In my experience, the pre-requisites are basically the same. So, I think this note makes sense!

[nightblade9]

Thank you! I hope this will make it easier for more non-C++ developers (like me) to get into building GDExtensions!

[akien-mga]

Thanks! And congrats for your first merged godot-cpp contribution 🎉

[dsnopek]

Cherry-picking to 4.2 in #1372

[dsnopek]

Cherry-picking to 4.1 in #1373