[Request] Add/create CONTRIBUTING.md file and guide for future contributors

[FaultyFunctions]

I would love to contribute to LDtk but I have never worked with Haxe before. I'd consider myself an intermediate dev and I've contributed to various open source JavaScript projects, but working my way around LDtk is a bit confusing for me for some reason. I'd love there to be some sort of guide for contributors on navigating their way around the project structure and how things are set up. This would make the friction of contributing to LDtk easier for people in the future.