Update NBD server usage documentation

[nroach44]

Pull Request checklist

Note: these are not necessarily requirements

• [ ❌ ] I reformatted the code with clang-format
• [ ❌ ] I checked to make sure my submission worked
• [ ✅ ] I am the author of submission or have permission from the original author
• [ ❌ ] Requires update of the PS2SDK or other dependencies
• [ ❌ ] Others (please specify below)
• [ ✅ ] 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.

[bignaux]

"server to share the internal hard drive, instead of HDL server."
In fact you can use NBD to share more than hdd actually ( https://bignaux.github.io/lwNBD/plugins/atad/lwnbd-atad-plugin.html ) , if i remembered, this is already merged actually :

• https://bignaux.github.io/lwNBD/plugins/mcman/lwnbd-mcman-plugin.html
• https://bignaux.github.io/lwNBD/plugins/memory/lwnbd-memory-plugin.html

Others plugins like audio stream or remote console are coming when i find energy. I made very important progress on that stuff.