Update quick-install.md

doc/quick-install.md

 
 ```sh
 add-apt-repository 'deb http://download.jitsi.org/nightly/deb unstable/'
+wget -qO - https://download.jitsi.org/nightly/deb/unstable/archive.key | apt-key add -
 ```
 
 add-apt-repository is in the default Ubuntu install and is available for both Ubuntu and Debian, but if it's not present, either install it with
 ```sh
 apt-get -y install software-properties-common
 add-apt-repository 'deb http://download.jitsi.org/nightly/deb unstable/'
+wget -qO - https://download.jitsi.org/nightly/deb/unstable/archive.key | apt-key add -
 ```
 
 or add the repository by hand with
 
 ```sh
 echo 'deb http://download.jitsi.org/nightly/deb trusty unstable/' >> /etc/apt/sources.list
+wget -qO - https://download.jitsi.org/nightly/deb/unstable/archive.key | apt-key add -
 ```
 
 ### Update the package lists