doc: Add documentation for Ubunut 18.04

Signed-off-by: Martin Winter <mwinter@opensourcerouting.org>

diff --git a/doc/Building_FRR_on_Ubuntu1804.md b/doc/Building_FRR_on_Ubuntu1804.md
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+Building FRR on Ubuntu 18.04LTS from Git Source
+===============================================
+
+Install required packages
+-------------------------
+
+Add packages:
+
+    apt-get install git autoconf automake libtool make gawk libreadline-dev \
+       texinfo pkg-config libpam0g-dev libjson-c-dev bison flex \
+       python-pytest libc-ares-dev python3-dev libsystemd-dev python-ipaddr
+
+Get FRR, compile it and install it (from Git)
+---------------------------------------------
+
+**This assumes you want to build and install FRR from source and not
+using any packages**
+
+### Add frr groups and user
+
+    sudo groupadd -g 92 frr
+    sudo groupadd -r -g 85 frrvty
+    sudo adduser --system --ingroup frr --home /var/run/frr/ \
+       --gecos "FRR suite" --shell /sbin/nologin frr
+    sudo usermod -a -G frrvty frr
+
+### Download Source, configure and compile it
+(You may prefer different options on configure statement. These are just
+an example.)
+
+    git clone https://github.com/frrouting/frr.git frr
+    cd frr
+    ./bootstrap.sh
+    ./configure \
+        --prefix=/usr \
+        --enable-exampledir=/usr/share/doc/frr/examples/ \
+        --localstatedir=/var/run/frr \
+        --sbindir=/usr/lib/frr \
+        --sysconfdir=/etc/frr \
+        --enable-pimd \
+        --enable-watchfrr \
+        --enable-ospfclient=yes \
+        --enable-ospfapi=yes \
+        --enable-multipath=64 \
+        --enable-user=frr \
+        --enable-group=frr \
+        --enable-vty-group=frrvty \
+        --enable-configfile-mask=0640 \
+        --enable-logfile-mask=0640 \
+        --enable-rtadv \
+        --enable-fpm \
+        --enable-systemd=yes \
+        --with-pkg-git-version \
+        --with-pkg-extra-version=-MyOwnFRRVersion
+    make
+    make check
+    sudo make install
+
+### Create empty FRR configuration files
+
+    sudo install -m 755 -o frr -g frr -d /var/log/frr
+    sudo install -m 775 -o frr -g frrvty -d /etc/frr
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/zebra.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/bgpd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ospfd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ospf6d.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/isisd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ripd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ripngd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/pimd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ldpd.conf
+    sudo install -m 640 -o frr -g frr /dev/null /etc/frr/nhrpd.conf
+    sudo install -m 640 -o frr -g frrvty /dev/null /etc/frr/vtysh.conf
+
+### Enable IPv4 & IPv6 forwarding
+
+Edit `/etc/sysctl.conf` and uncomment the following values (ignore the 
+other settings)
+
+    # Uncomment the next line to enable packet forwarding for IPv4
+    net.ipv4.ip_forward=1
+
+    # Uncomment the next line to enable packet forwarding for IPv6
+    #  Enabling this option disables Stateless Address Autoconfiguration
+    #  based on Router Advertisements for this host
+    net.ipv6.conf.all.forwarding=1
+
+### Enable MPLS Forwarding (with Linux Kernel >= 4.5)
+
+Edit `/etc/sysctl.conf` and the following lines. Make sure to add a line 
+equal to `net.mpls.conf.eth0.input` or each interface used with MPLS
+
+    # Enable MPLS Label processing on all interfaces
+    net.mpls.conf.eth0.input=1
+    net.mpls.conf.eth1.input=1
+    net.mpls.conf.eth2.input=1
+    net.mpls.platform_labels=100000
+
+### Add MPLS kernel modules
+
+Add the following lines to `/etc/modules-load.d/modules.conf`:
+
+    # Load MPLS Kernel Modules
+    mpls-router
+    mpls-iptunnel
+
+**Reboot** or use `sysctl -p` to apply the same config to the running system
+
+### Install the systemd service (if rebooted from last step, change directory back to frr directory)
+
+    sudo install -m 644 tools/frr.service /etc/systemd/system/frr.service
+    sudo install -m 644 tools/etc/default/frr /etc/default/frr
+    sudo install -m 644 tools/etc/frr/daemons /etc/frr/daemons
+    sudo install -m 644 tools/etc/frr/daemons.conf /etc/frr/daemons.conf
+    sudo install -m 644 tools/etc/frr/frr.conf /etc/frr/frr.conf
+    sudo install -m 644 -o frr -g frr tools/etc/frr/vtysh.conf /etc/frr/vtysh.conf
+
+### Enable daemons 
+
+Edit `/etc/frr/daemons` and change the value from "no" to "yes" for those daemons you want to start by systemd.  
+For example.
+
+    zebra=yes
+    bgpd=yes
+    ospfd=yes
+    ospf6d=yes
+    ripd=yes
+    ripngd=yes
+    isisd=yes
+
+### Enable the systemd service
+-  systemctl enable frr
+
+### Start the systemd service
+-  systemctl start frr
+- use `systemctl status frr` to check its status.