Example of simple state machine

[jkaniuka]

Hi :wave:, I'm happy to see that SMACH is now available in ROS 2. A number of tutorials describing SMACH's features have been created for ROS 1 (you can find them here). Migrating all of these tutorials to ROS 2, would require a lot of work and I don't know if it is necessary. I believe it would be enough to add in ReadMe an example of a simple state machine in ROS 2. The example I added is the same as in the tutorial for ROS 1. I think it will encourage people who find this repository to use SMACH :wink: