Add a note for installing xdg-utils as a dependency of the websearch plugin

[dysthesis]

The websearch plugin relies on xdg-open, which does not always come preinstalled on every system. This should therefore be mentioned on the README.

[Kirottu]

Sorry for the late reply, but I would change it to be a more generic note for installing xdg-utils instead of being the instructions specific to NixOS.