Align license with `rslidar_msg`

[Timple]

The license of the package rslidar_msg is not in line with this package. In fact it is missing.

Little developers seem to be subscribed to the rslidar_msg, but I've opened up a pull-request to fix this: https://github.com/RoboSense-LiDAR/rslidar_msg/pull/1

[RS-Alex]

Thanks for the feedback, we will add the license shortly

[ronzheng1006]

We have merge this PR. Thanks.

[Timple]

Thank you!