documenting rocker and only pulling it from pip conditionally

README.md

@@ -18,6 +18,7 @@ This demo has been tested on Ubuntu Xenial (16.04)
 * [Docker](https://www.docker.com/get-docker)
 * [nvidia-docker2](https://github.com/nvidia/nvidia-docker/wiki/Installation-(version-2.0))
 * The current user is a member of the docker group or other group with docker execution rights.
+* [rocker](https://github.com/osrf/rocker)
 
 # Recommended
 

run_demo.bash

@@ -14,10 +14,12 @@ do
     sleep 1
 done
 
-mkdir -p /tmp/car_demo_rocker_venv
-python3 -m venv /tmp/car_demo_rocker_venv
-. /tmp/car_demo_rocker_venv/bin/activate
-pip install -U git+https://github.com/osrf/rocker.git
-
+if ! [ -x "$(command -v git)" ]; then
+    echo "Rocker not found pulling from pip"
+    mkdir -p /tmp/car_demo_rocker_venv
+    python3 -m venv /tmp/car_demo_rocker_venv
+    . /tmp/car_demo_rocker_venv/bin/activate
+    pip install -U git+https://github.com/osrf/rocker.git
+fi
 
 rocker --nvidia --x11 --devices /dev/input/js0 /dev/input/js1 -- osrf/car_demo