Copy command-line documentation into `USAGE.md` & add documentation for `-additionaldir`

[Jayman2000]

Pull Request Type

• [ ] GitHub Workflow changes
• [x] Documentation or Wiki changes
• [ ] Build and Dependency changes
• [ ] Runtime changes
  • [ ] Render changes
  • [ ] Audio changes
  • [ ] Input changes
  • [ ] Network changes
  • [ ] Other changes

Description

This pull request takes the table of command-line options that’s on the wiki and merges it with the one in USAGE.md. It also adds documentation for the -additionaldir command-line option.

Related Issues

This is related #618, but it doesn’t fully fix that issue.

Checklist

• [x] I have tested my changes locally and verified that they work as intended.
• [x] I have documented any new or modified functionality.
• [x] I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• [x] I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.

Additional Comments