remove deprecated apt-key from installation setup (#354)

README.md

@@ -34,7 +34,7 @@ the project using the link in the header.
 To setup the package repository, run these commands:
 
 ```sh
-$ wget -qO - https://packagecloud.io/shiftkey/desktop/gpgkey | sudo apt-key add -
+$ wget -qO - https://packagecloud.io/shiftkey/desktop/gpgkey | sudo tee /etc/apt/trusted.gpg.d/shiftkey-desktop.asc > /dev/null
 $ sudo sh -c 'echo "deb [arch=amd64] https://packagecloud.io/shiftkey/desktop/any/ any main" > /etc/apt/sources.list.d/packagecloud-shiftky-desktop.list'
 $ sudo apt-get update
 ```