Add safety section to README

[nepfaff]

It is very easy to blow the iiwa fuses by sending the wrong commands to the robot. We should have a README section that guides users on how to prevent this:

• Always retime with Toppra before sending trajectories to the robot
• Always make sure that the commands sent to the robot look reasonable in simulation before sending them to the robot (never develop in hardware directly)
• Always execute trajectories at slow speeds first