add README.md in rust example to address readers to waku-rust-binding…

…s repo

examples/rust/README.md

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+This represents a very simple example on how to integrate the `libwaku` library in Rust, and then, only a few `libwaku` functions are being wrapped.
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+In [waku-rust-bindings](https://github.com/waku-org/waku-rust-bindings) you will find a complete Waku integration in Rust.
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