Merge pull request dora-rs#431 from dora-rs/cxx-ros2-readme

Create README.md for cxx-ros2-example

examples/c++-ros2-dataflow/README.md

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+# `cxx-ros2-dataflow` Example
+
+This c++ example shows how to publish/subscribe to both ROS2 and Dora. The dataflow consists of a single node that sends random movement commands to the [ROS2 `turtlesim_node`](https://docs.ros.org/en/iron/Tutorials/Beginner-CLI-Tools/Introducing-Turtlesim/Introducing-Turtlesim.html).
+
+## Setup
+
+This examples requires a sourced ROS2 installation.
+
+- To set up ROS2, follow the [ROS2 installation](https://docs.ros.org/en/iron/Installation.html) guide.
+- Don't forget to [source the ROS2 setup files](https://docs.ros.org/en/iron/Tutorials/Beginner-CLI-Tools/Configuring-ROS2-Environment.html#source-the-setup-files)
+- Follow tasks 1 and 2 of the [ROS2 turtlesim tutorial](https://docs.ros.org/en/iron/Tutorials/Beginner-CLI-Tools/Introducing-Turtlesim/Introducing-Turtlesim.html#id3)
+  - Install the turtlesim package
+  - Start the turtlesim node through `ros2 run turtlesim turtlesim_node`
+
+## Running turtlesim example
+
+
+From terminal 1 , sourcing the ROS2 installation and start ROS2 turtlesim window
+```
+source /opt/ros/galactic/setup.bash
+ros2 run turtlesim turtlesim_node
+```
+
+From terminal 2 from dora folder. Note the source command here is necessary as this allow ROS2 message types to be found and compile dynamically.
+```
+source /opt/ros/galactic/setup.bash
+cargo run --example cxx-ros2-dataflow --features ros2-examples
+```
+And you will see the turtle move a few steps.