Update README with full instructions

[richardrl]

You will get permissions errors unless you modify create_urdf.sh with your username and user id and gid like so:

docker build -t urdf_creation \
    --build-arg USERNAME=<myuser> \
    --build-arg USER_UID=<myuid> \
    --build-arg USER_GID=<mygid> \
    ./.docker

echo 

docker run -u <myuid> \
    -v $(pwd):/workspaces/src/franka_description \
    -w /workspaces/src/franka_description \
    urdf_creation \
    .docker/create_urdf.entrypoint.sh $*

[gomezgu]

Hi richardlr,

Thank you for your feedback. The user uid was hardcoded to be 1001 and that was probably causing the mismatch. We will fix the issue.