Add gazebo model path to atom examples documentation

[brunofavs]

This issue is a follow-up to issue #843 , where I had a problem getting the patterns to load in gazebo.

It's necessary to add this line to .bashrc/.zshrc to run the calibration studio world properly :

export GAZEBO_MODEL_PATH="rospack find atom_worlds/pattern/models:${GAZEBO_MODEL_PATH}"

It turned out there was a detail missing in the atom_examples documentation that used to be in the installation of each robotic system.

[miguelriemoliveira]

Thanks. I will pick it up when I have time.