Update README with newest features / architecture - #36

Can you elaborate on this section?

Make sure that you have built and loaded (`source install/setup.bash`) the extension.

As a newcomer to the tool, from the usage described in: https://github.com/ros2-rust/ros2_rust?tab=readme-ov-file#sounds-great-how-can-i-try-this-out

It seems like we source the workspace of Rust crates after we build the ROS workspace. Is my understanding incorrect?

I would expect that we don't have to source anything from colcon-ros-cargo since it's a colcon extension

It's perhaps a bit redundant and more of a general direction about how to install colcon extensions, since for now there are no debian packages this can only be pip installed (with the scary --break-system-packages flag) or put in a colcon workspace and colcon build.
I gave it a quick try and indeed if I have a workspace with this colcon extension and a package that would be built by it (something that is identified as a ros.ament_cargo package named example) the following seem to be true:

• Doing colcon build --packages-up-to example doesn't build this colcon extension, which makes sense since it's not an explicit dependency I guess.
• Doing colcon build --packages-select colcon-ros-cargo, followed by colcon build --package-select example does not correctly identify the package, and colcon still reports that it doesn't know how to handle a ros.ament_cargo package.
• Doing a colcon build --packages-select colcon-ros-cargo, followed by a source install/setup.bash, followed by a colcon build --packages-select example correctly works, suggesting that indeed colcon extensions need to be built and sourced before they can be used.