Add Backports hint to the Debian Download section.

Something like:
Debian GNU/Linux:
Just sudo apt-get install gajim
If you want to install the latest stable:
Remove Gajim:**
sudo apt-get remove gajim
Add the Debian backports repository:
For stretch:
sudo sh -c "echo deb http://ftp.debian.org/debian stretch-backports main > /etc/apt/sources.list.d/backports.list"
For For jessie:
sudo sh -c "echo deb http://ftp.debian.org/debian jessie-backports main > /etc/apt/sources.list.d/backports.list"
Update the repository:
sudo apt-get update
And install Gajim by doing:
sudo apt-get install gajim
A daily package from git is also built. To use it:
Remove Gajim:
sudo apt-get remove gajim
Add the GPG key:
wget -q https://gajim.org/gajim.key -O - | apt-key add -
Add the repository:
sudo sh -c "echo deb ftp://ftp.gajim.org/debian unstable main > /etc/apt/sources.list.d/gajim.list"
Update the repository:
sudo apt-get update
And install Gajim by doing:
sudo apt-get install gajim-nightly