Update NBD server usage documentation

[nroach44]

Pull Request checklist

Note: these are not necessarily requirements

• [ :x: ] I reformatted the code with clang-format
• [ :x: ] I checked to make sure my submission worked
• [ :white_check_mark: ] I am the author of submission or have permission from the original author
• [ :x: ] Requires update of the PS2SDK or other dependencies
• [ :x: ] Others (please specify below)
• [ :white_check_mark: ] Verified the readme read OK in GitHub's viewer

Pull Request description

Updates the NBD server section of README.MD to read better, and explain how to use it.