Add example of using LHAPDF

[matthewfeickert]

LHAPDF is installed in the Docker images

MadGraph5_aMC-NLO/docker/debian/Dockerfile

Lines 82 to 96 in 7678e88

	# Install LHAPDF
	ARG LHAPDF_VERSION=6.3.0
	RUN mkdir /code && \
	cd /code && \
	wget https://lhapdf.hepforge.org/downloads/?f=LHAPDF-${LHAPDF_VERSION}.tar.gz -O LHAPDF-${LHAPDF_VERSION}.tar.gz && \
	tar xvfz LHAPDF-${LHAPDF_VERSION}.tar.gz && \
	cd LHAPDF-${LHAPDF_VERSION} && \
	./configure --help && \
	export CXX=$(which g++) && \
	export PYTHON=$(which python) && \
	./configure \
	--prefix=/usr/local/venv && \
	make -j$(($(nproc) - 1)) && \
	make install && \
	rm -rf /code

which allows for users to download LHAPDF sets

$ docker run --rm -ti scailfin/madgraph5-amc-nlo:mg5_amc3.3.2
root@898ccfb286ce:~/data# lhapdf get NNPDF23_lo_as_0130_qed
Unable to download /cvmfs/sft.cern.ch/lcg/external/lhapdfsets/current/NNPDF23_lo_as_0130_qed.tar.gz
NNPDF23_lo_as_0130_qed.tar.gz:    26.3 MB [100.0%] 
root@898ccfb286ce:~/data# lhapdf show NNPDF23_lo_as_0130_qed
NNPDF23_lo_as_0130_qed
======================
LHAPDF ID: 247000
Version: 1
LO QCD + LO QED PDF set, alphas(MZ)=0.130. mem=0 => average on replicas; mem=1-100 => PDF replicas
Number of members: 101
Error type: replicas

root@898ccfb286ce:~/data#

As pointed out by @lukasheinrich, it would probably be helpful to document this with a short note in the README.