message: adds safety distances documentation.
diff: <?xml version="1.0" encoding="utf-8"?>
  <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="hand">
  + <!-- safety_distance: Minimum safety distance in [m] by which the collision volumes are expanded and which is enforced during robot motions -->
  <xacro:macro name="hand" params="connected_to:='' ns:='' rpy:='0 0 0' xyz:='0 0 0' safety_distance:='0'">
  <xacro:unless value="${connected_to == ''}">
  <joint name="${ns}_hand_joint" type="fixed">
  