Update README.md

Author: eel76

README.md

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 # Traits for C++
 
+[![traits CI](https://github.com/VolumeGraphics/traits/actions/workflows/cmake-multi-platform.yml/badge.svg)](https://github.com/VolumeGraphics/traits/actions/workflows/cmake-multi-platform.yml)
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 Define shared behavior in a non-intrusive way while preserving value semantics.
 
 This library is inspired by [Rust Traits](https://doc.rust-lang.org/book/ch10-02-traits.html) and previous projects like [Dyno](https://github.com/ldionne/dyno).