Copy this folder to the gnome-shell extension folders, typically under .local/share/gnome-shell/extensions.
Using a terminal, you can do the following sequence of commands :
$ git clone https://github.com/laerne/desaturate_all.git $ mv desaturate_all ".local/share/gnome-shell/extensions/desaturate_all@nicolas.brack.mail.be"
Then restart gnome by pressing ALT+F2, then entering r and validating with RETURN.
You can now enable the extension by browsing to https://extensions.gnome.org/local .