Add Linux instructions

INSTALL_LINUX.md

+# Instructions to install libre.sh on linux with Systemd  
+
+## Recommendation  
+- Systemd (debian 8 or debian 9, CentOS 7 ...)  
+
+# Installation  
+Where basicly reproduce what the user_data do for us.  
+
+as root  
+
+# configure sshd  
+Don't forget to create the user core and adding your ssh key before
+You could also remove AllowUsers core or/and change the username.  
+  
+> cat > /etc/ssh/sshd_config <<EOF  
+UsePrivilegeSeparation sandbox  
+Subsystem sftp internal-sftp  
+PermitRootLogin no  
+AllowUsers core  
+PasswordAuthentication no  
+ChallengeResponseAuthentication no  
+EOF  
+> chmod 600 /etc/ssh/sshd_config  
+> systemctl restart sshd  
+
+# add kernel parameter  
+  
+> cat > /etc/sysctl.d/libresh.conf <<EOF  
+fs.aio-max-nr=1048576  
+vm.max_map_count=262144  
+EOF  
+> chmod 644 /etc/sysctl.d/libresh.conf
+> sysctl -p  
+
+# define Localhost
+  
+> cat > /etc/hosts <<EOF  
+127.0.0.1 localhost  
+255.255.255.255 broadcasthost  
+::1 localhost  
+EOF  
+
+# define envrionment  
+  
+> cat > /etc/environment <<EOF  
+NAMECHEAP_URL="namecheap.com"  
+NAMECHEAP_API_USER="pierreo"  
+NAMECHEAP_API_KEY=  
+IP="curl -s http://icanhazip.com/"  
+FirstName="Pierre"  
+LastName="Ozoux"  
+Address=""  
+PostalCode=""  
+Country="Portugal"  
+Phone="+351.967184553"  
+EmailAddress="pierre@ozoux.net"  
+City="Lisbon"  
+CountryCode="PT"  
+BACKUP_DESTINATION=root@xxxxx:port  
+MAIL_USER=  
+MAIL_PASS=  
+MAIL_HOST=mail.indie.host  
+MAIL_PORT=587  
+EOF  
+
+# install docker-compose  
+  
+*Remark I did a variante to find the last version of DockerCompose and download it*
+> mkdir -p /opt/bin &&\  
+> dockerComposeVersion=$(curl -s https://api.github.com/repos/docker/compose/releases/latest|grep tag_name|cut -d'"' -f4) &&\  
+> curl -L https://github.com/docker/compose/releases/download/$dockerComposeVersion/docker-compose-`uname -s`-`uname -m` > /opt/bin/docker-compose &&\  
+> chmod +x /opt/bin/docker-compose  
+
+# install Libre.sh  
+  
+> git clone https://github.com/indiehosters/libre.sh.git /libre.sh &&\  
+> mkdir /{data,system} &&\  
+> mkdir /data/trash &&\  
+> cp /libre.sh/unit-files/* /etc/systemd/system && systemctl daemon-reload &&\  
+> systemctl enable web-net.service &&\  
+> systemctl start web-net.service &&\  
+> cp /libre.sh/utils/* /opt/bin/  
+
+# add /opt/bin path  
+  
+> cat > /etc/profile.d/libre.sh <<EOF  
+export PATH=$PATH:/opt/bin  
+EOF  
+chmod 644 /etc/profile.d/libre.sh