Update README.md

Author: git4dcc

README.md

@@ -3,7 +3,7 @@
 [![Kicad_Libs](https://img.shields.io/badge/Kicad_Libs-29C7FF)](https://github.com/git4dcc/RTB_SamacSys)
 [![Apache License 2.0](https://img.shields.io/badge/license-Apache%20License%202.0-lightgray)](https://www.apache.org/licenses/LICENSE-2.0)
 
-The D21 decoder is a double sided mobile decoder with Next18 connector. The decoder is designed to integrate into the [RTB](https://rtb4dcc.de/concept/) digital control infrastructure.
+The [D21](https://rtb4dcc.de/hardware/decoder/d21/) decoder is a double sided mobile decoder with Next18 connector. The decoder is designed to integrate into the [RTB](https://rtb4dcc.de/concept/) digital control infrastructure.
 
 <details>
 <summary>See also</summary>