Update README.md

README.md

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 > **Warning**
 > This project is in early development, it does however work with real sats! Always use amounts you don't mind losing.
-[![crates.io](https://img.shields.io/crates/v/cdk.svg)](https://crates.io/crates/cdk)
+[![crates.io](https://img.shields.io/crates/v/cdk.svg)](https://crates.io/crates/cdk) [![Documentation](https://docs.rs/cdk/badge.svg)](https://docs.rs/cdk)
 
 # Cashu Development Kit