message: docs: add documentation for installing xsel. fixes
diff: @@ -64,6 +64,14 @@ We also recommend going into *System Preferences -> Keyboard* and increasing the
  VSCodeVim will take over your control keys, just like real vim, so you get the _full_ vim experience. This behaviour can be adjusted with the [`useCtrlKeys`](#vimusectrlkeys) and [`handleKeys`](#vimhandlekeys) settings.
  +### Linux setup
  +
  +If you have configured `vim.useSystemClipboard: "true"`, we rely on [clipboardy](https://github.com/sindresorhus/clipboardy) for cross-platform copy/paste operations. This library is dependent on `xsel`:
  +
  +```
  +apt install xsel
  +```
  +
  ## Settings
  ### Quick example settings
  