Add link to Demos repo in README

[peredwardsson]

Feature request

Background

I wanted to make a LifecycleNode in c++ and visited the GitHub page to learn about the interface and what I need in order to fulfil it. I had issues finding that information here, and I had more issues trying to learn from the links in the README. In lack for a clear tutorial, a good demo can suffice, I thought, and went looking, but could still not find anything helpful. I resorted to writing on ROS Answers, where I was linked to the Demo repo. This is what I was looking for.

Feature description

In order to make it easier for someone to use the code provided in this package, the README should link to the demos repository, since the demo repository contains demos of how to use this code.

Implementation considerations

It wouldn't be wrong to have an examples folder in this repo either, I think, but an easy solution to make LifecycleNodes (in particular) more approachable would be to link to somewhere they are used.

[fujitatomoya]

this sounds reasonable to me.

IMO, as architecture

• we could have dependency to https://docs.ros.org to navigate the user examples and tutorials described in ROS 2 documentation.
• so that we can focus the effort on documentation for https://github.com/ros2/ros2_documentation, instead of complicated dependencies.
• for each packages, we could have more guidance for examples and tutorials described in ROS 2 documentation.