doc: Adding Doc for building on Debian 9

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

doc/Building_FRR_on_Debian9.md

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+Building FRR on Debian 9 from Git Source
+========================================
+
+Install required packages
+-------------------------
+
+Add packages:
+
+    sudo apt-get install git autoconf automake libtool make \
+      libreadline-dev texinfo libjson-c-dev pkg-config bison flex \
+      python-pip libc-ares-dev python3-dev python-pytest
+
+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 addgroup --system --gid 92 frr
+    sudo addgroup --system --gid 85 frrvty
+    sudo adduser --system --ingroup frr --home /var/run/frr/ \
+       --gecos "FRR suite" --shell /bin/false 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
+    git checkout stable/3.0
+    ./bootstrap.sh
+    ./configure \
+        --enable-exampledir=/usr/share/doc/frr/examples/ \
+        --localstatedir=/var/run/frr \
+        --sbindir=/usr/lib/frr \
+        --sysconfdir=/etc/frr \
+        --enable-vtysh \
+        --enable-isisd \
+        --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-ldpd \
+        --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 IP & 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
+
+**Reboot** or use `sysctl -p` to apply the same config to the running system
+
+### Troubleshooting
+
+**Local state directory**
+
+The local state directory must exist and have the correct permissions applied
+for the frrouting daemons to start.  In the above ./configure example the
+local state directory is set to /var/run/frr (--localstatedir=/var/run/frr)
+Debian considers /var/run/frr to be temporary and this is removed after a
+reboot.
+
+When using a different local state directory you need to create the new
+directory and change the ownership to the frr user, for example:
+
+    mkdir /var/opt/frr
+    chown frr /var/opt/frr
+
+**Shared library error**
+
+If you try and start any of the frrouting daemons you may see the below error
+due to the frrouting shared library directory not being found:
+
+    ./zebra: error while loading shared libraries: libfrr.so.0: cannot open shared object file: No such file or directory
+
+The fix is to add the following line to /etc/ld.so.conf which will continue to
+reference the library directory after the system reboots.  To load the library
+directory path immediately run the ldconfig command after adding the line to
+the file eg:
+
+    echo include /usr/local/lib >> /etc/ld.so.conf
+    ldconfig