Add Clef UI to README.md (#17763)

cmd/clef/README.md

@@ -875,3 +875,4 @@ There are a couple of implementation for a UI. We'll try to keep this list up to
 | QtSigner| https://github.com/holiman/qtsigner/| Python3/QT-based| :+1:| :+1:| :+1:| :+1:| :+1:| :x: |  :+1: (partially)|
 | GtkSigner| https://github.com/holiman/gtksigner| Python3/GTK-based| :+1:| :x:| :x:| :+1:| :+1:| :x: |  :x: |
 | Frame | https://github.com/floating/frame/commits/go-signer| Electron-based| :x:| :x:| :x:| :x:| ?| :x: |  :x: |
+| Clef UI| https://github.com/kyokan/clef-ui| Golang/QT-based| :+1:| :+1:| :x:| :+1:| :+1:| :x: |  :+1: (approve tx only)|