Fixed the broken link to CONTRIBUTING.md (#737)

Author: hxrshxz

README.md

@@ -49,7 +49,7 @@ To learn more on how to use Lumino, check out the documentation: https://lumino.
 
 ## Development
 
-See [CONTRIBUTING.md](./CONTRIBUTING.md) to know how to contribute and set up
+See [CONTRIBUTING.md](https://github.com/jupyterlab/lumino/blob/main/CONTRIBUTING.md) to know how to contribute and set up
 a development environment.
 
 ## Learning Resources