Add a note on use in ROS 2 in README.rst

[traversaro]

I am not familiar with maintenance of ROS 2, and I was a bit confused by the user of catkin_pkg in ROS 2 context (see for example https://docs.ros.org/en/rolling/How-To-Guides/Core-maintainer-guide.html#source-release), as I always associated catkin to ROS 1.

However, from what I understand this is how things I done, so I added a note on this to the repo README to clarify this for non-expert users (like me). Feel free to bikeshed the exact wording or the location of this documentation (xref: #343).