Explain how to install tails-installer from our APT repository

For the first testing sessions it won't be in Debian backports.

wiki/src/install/inc/steps/install_tails_installer.inline.mdwn

@@ -17,12 +17,23 @@ Installer</span>.</p>
 
 </div>
 
+<!--
+
 1. If you are running Debian, execute the following command to add the
    [backports](http://backports.debian.org/) repository to your system:
 
        echo "deb http://http.debian.net/debian jessie-backports main" | \
             sudo tee /etc/apt/sources.list.d/jessie-backports.list
 
+-->
+
+1. If you are running Debian, execute the following command to add the
+   Tails APT repository to your system:
+
+       wget -O - https://labs.riseup.net/code/attachments/download/1052/tails-apt.key | sudo apt-key add -
+       echo "deb http://deb.tails.boum.org/ feature-jessie main" | \
+            sudo tee /etc/apt/sources.list.d/tails.list
+
 1. Installer the <span class="code">tails-installer</span> package:
 
        sudo apt update