Build doc: install pigz for faster Vagrant box creation

pigz, i.e. "Parallel Implementation of GZip", takes advantage of multiple
processors and multiple cores when compressing data.

Recent vagrant-libvirt's create_box.sh uses pigz if it's available, which
speeds up the creation of our Vagrant build box on multi-core machines.

wiki/src/contribute/build.mdwn

@@ -34,6 +34,7 @@ To build Tails you need:
            libvirt-daemon-system \
            dnsmasq-base \
            ebtables \
+           pigz \
            qemu-system-x86 \
            qemu-utils \
            vagrant \