Add a README.

[Esper89]

I figured it would be a good idea to have info on how to build the modloader on Linux available outside the Discord server, so I wrote a README.

[Esper89]

Wow, I'm impressed that adding one markdown file made the CI fail. 🤣

[Esper89]

There we go. 😌

[CJendantix]

You can have temporary environment variables that only last for one command, rather than staying in the environment, by prefixing the commands with the variables rather than exporting them beforehand. I recommend changing

export USE_PRECOMPILED_CPP_LOADER=1
export USE_PREBUILT_ASSETS=1
cargo build --release

to
USE_PRECOMPILED_CPP_LOADER=1 USE_PREBUILT_ASSETS=1 cargo build --release