added updated install instructions to correctly initialize nanocernlib submodule

README.md

@@ -34,7 +34,7 @@ Ensure you have
 Once you have all dependencies installed, you are ready to build pythia6m
 ```bash
 mkdir -p ${PREFIX}/src && cd ${PREFIX}/src
-git clone ${REPO}
+git clone --recurse-submodules ${REPO}
 mkdir -p pythia6m/BUILD && cd pythia6m/BUILD
 cmake .. -DCMAKE_INSTALL_PREFIX=${PREFIX} -DCMAKE_PREFIX_PATH=${PREFIX}
 make -j4