Add installation instuctions (see 86box issue #6527)

[TheSystemGuy1337]

Description:

This tripped me up the first time so to prevent confusion, I added some installation instructions to the README.

Checklist:

• [Not needed] I have opened a pull request in the main 86Box repository.
• [X ] I have not added any unrelated files to this pull request.
• [X ] My commit(s) follow the committing guidelines.

Your 86Box pull request:

N/A - I'm just adding instructions here as I got tripped up by that and there doesn't appear to be any installation instructions for the FDD sounds.

[StarWolf3000]

My structure on Arch is as follows:

• 86Box main repo clone: ~/86Box/
• binary: ~/86Box/build/src/86Box
• roms repo clone: ~/.local/share/86Box/roms/
• assets repo clone: ~/.local/share/86Box/assets/

[TheSystemGuy1337]

This PR draft seems to have stalled. I will close it in a week if nothing happens.

[TheSystemGuy1337]

This is going nowhere, closing

[OBattler]

I didn't merge it because it never made it past the draft stage.