Each loadout has its own folder and a readme file. Look at the readme file to know what general and specific items are in the loadout.
Just download the whole repo and extract what you want, or hand pick the loadout files and paste their contents into your mission. Or if you're bored you could write a little CLI tool to do that for you, and you just call arc-loadout blufor usmc-woodland, but that's just overkill. Or is it...
To contribute to this repo, you'll need to install Git and submit a pull request. There's plenty of tutorials and resources online to teach you that. When you're ready to write the readme for your loadout, take a look at the example loadout for the format.