update comment and README.md
Merge pull request #21 from YoheiKakiuchi/add_description

Add description
#25: Merge branch 'master' into garaemon-fix-twice-compilation-for-urdf-model-id-25
#25: depends hrpsys_gazebo_tutorials on hrpsys_ros_bridge_tutorials. and
executes each urdf generation after compiling hrpsys_gazebo_tutorials
by using ${compile_robots} targets