Add README files for rclrs and rosidl_runtime_rs (#273)

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Author: nnmm

rosidl_runtime_rs/README.md

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+# Common types and traits for ROS 2 messages in Rust
+
+ROS 2 is a popular open source robotics framework, used in a variety of fields (self-driving cars, drones, humanoid robots, etc.). `rosidl_runtime_rs` is a library that is mainly used by generated code for ROS 2 messages.
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+Please see the docs in the [`ros2_rust` repo](https://github.com/ros2-rust/ros2_rust).