add helpful hints for ubuntu users

README.ubutntu

+
+The Debian and Ubuntu GNOME packages don't have reasonable defaults,
+so you have to give ./configure every path manually.
+
+  ./configure \
+	--prefix=/usr --localstatedir=/etc --sysconfdir=/etc \
+	--sharedstatedir=/var/lib --libexecdir=/usr/lib \
+	--infodir=/usr/share/info --mandir=/usr/share/man \
+	--with-pppd-plugin-dir=/usr/lib/pppd/2.4.5/
+
+You'll need at least the following installed:
+
+  sudo apt-get install xl2tpd openswan
+