[documentation] Howto clipboard

[DrYak]

Context

The roadmap in the main readme still lists clipboard unchecked:

• Clipboard support

While technically true, as lan-mouse's own protocol doesn't (and can't easily for technical reasons around UDP realtime communications) carry clipboard content, in practice clipboard sharing works like a charm (see #105) thanks to hooks, and external tools such as wl-clipboard/wl-clipboard-rs and xclip (for clipboard management) and SSH forced commands (for encryption and security) or Syncthing (keeping clipboard history synced)

TODO

Once I get more of that elusive resource called "time" I should write a clear documentation on how we achieve this, including sample scripts.
(On my side: at least for Wayland and X11 machines. Maybe someone could do Windows or Mac OS based on their experience ?).
And we should link that from the roadmap.