mod launch options and add "Troubleshooting" section in doc/index.rst

[y-yosuke]

mod launch options for a /force topic publishing error with serial communication errors and add Troubleshooting section in document doc/index.rst.

The issue is #48.

[k-okada]

@y-yosuke travis failing

-- run_tests.py: execute commands
  /usr/bin/cmake -E make_directory /tmp/ws/test_results/dynpick_driver
  /opt/ros/melodic/share/roslaunch/cmake/../scripts/roslaunch-check -o "/tmp/ws/test_results/dynpick_driver/roslaunch-check_launch_sample.launch.xml" "/tmp/ws/src/dynpick_driver/launch/sample.launch" 
checking /tmp/ws/src/dynpick_driver/launch/sample.launch
...writing test results to /tmp/ws/test_results/dynpick_driver/roslaunch-check_launch_sample.launch.xml
FAILURE:
[/tmp/ws/src/dynpick_driver/launch/sample.launch]:
	while processing /tmp/ws/src/dynpick_driver/launch/driver.launch:
Invalid <arg> tag: cannot override arg 'acquire_calibration', which has already been set. 

Arg xml is <arg name="acquire_calibration" value="true"/>

may be we need to set <arg name="acquire_calibration" default="true"/> ???