Move English documentation to folder en_US

doc/en_US/debian-installation.markdown

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+Kanboard Installation on Debian
+===============================
+
+Debian 8 (Jessie)
+-----------------
+
+Install Apache and PHP:
+
+```bash
+apt-get update
+apt-get install -y php5 php5-sqlite php5-gd unzip
+service apache2 restart
+```
+
+Install Kanboard:
+
+```bash
+cd /var/www/html
+wget https://kanboard.net/kanboard-latest.zip
+unzip kanboard-latest.zip
+chown -R www-data:www-data kanboard/data
+rm kanboard-latest.zip
+```
+
+Debian 7 (Wheezy)
+-----------------
+
+Install Apache and PHP:
+
+```bash
+apt-get update
+apt-get install -y php5 php5-sqlite php5-gd unzip
+```
+
+Install Kanboard:
+
+```bash
+cd /var/www
+wget https://kanboard.net/kanboard-latest.zip
+unzip kanboard-latest.zip
+chown -R www-data:www-data kanboard/data
+rm kanboard-latest.zip
+```
+
+Debian 6 (Squeeze)
+------------------
+
+Install Apache and PHP:
+
+```bash
+apt-get update
+apt-get install -y libapache2-mod-php5 php5-sqlite php5-gd unzip
+```
+
+Install Kanboard:
+
+```bash
+cd /var/www
+wget https://kanboard.net/kanboard-latest.zip
+unzip kanboard-latest.zip
+chown -R www-data:www-data kanboard/data
+rm kanboard-latest.zip
+```
+
+Notes
+-----
+
+Some features of Kanboard require that you run [a daily background job](cronjob.markdown).