[docs] remove link to Roadmap (fixes #6556)

fixes #6556

CONTRIBUTING.md

@@ -6,7 +6,7 @@ Your help is very valuable to make it better for everyone.
 
 ## How to Contribute
 
-- Check for the [Roadmap](https://github.com/microsoft/LightGBM/projects/1) and the [Feature Requests Hub](https://github.com/microsoft/LightGBM/issues/2302), and submit pull requests to address chosen issue. If you need development guideline, you can check the [Development Guide](https://github.com/microsoft/LightGBM/blob/master/docs/Development-Guide.rst) or directly ask us in Issues/Pull Requests.
+- Check the [Feature Requests Hub](https://github.com/microsoft/LightGBM/issues/2302), and submit pull requests to address chosen issue. If you need development guideline, you can check the [Development Guide](https://github.com/microsoft/LightGBM/blob/master/docs/Development-Guide.rst) or directly ask us in Issues/Pull Requests.
 - Contribute to the [tests](https://github.com/microsoft/LightGBM/tree/master/tests) to make it more reliable.
 - Contribute to the [documentation](https://github.com/microsoft/LightGBM/tree/master/docs) to make it clearer for everyone.
 - Contribute to the [examples](https://github.com/microsoft/LightGBM/tree/master/examples) to share your experience with other users.