Remove references to rclrs not being on crates.io

[esteve]

Fixes #334

[maspe36]

So it should be possible to build an rclrs node without having the entirety of ros2_rust in your workspace or an underlay?

[esteve]

@maspe36 I'm afraid I'm a bit lost here, is your comment about #380 ?

[maspe36]

I am specifically referring to this removed line

Note: Once rclrs is published on crates.io, it's not technically needed anymore to clone the ros2_rust repo, and this section will be modified to reflect that.

It sounds like once I run these commands,

cargo install --debug cargo-ament-build 
pip install git+https://github.com/colcon/colcon-cargo.git
pip install git+https://github.com/colcon/colcon-ros-cargo.git

I could write a node in rust that pulls rclrs from crates.io when I colcon build. Is that understanding correct?

[mxgrey]

I'd love to be wrong about this but I think for now you'd at least need to have rosild_generator_rs in your colcon workspace with the message packages that you need. If we can get that package on the build farm then I think the usage you're describing would work.