contributing: add implicit-hie gen-hie > hie.yaml note

This tool helps me for years.

Author: Anton-Latukha

docs/contributing/contributing.md

@@ -124,6 +124,12 @@ cradle:
 
 Also note that many subdirectories of a project are in itself are subprojects, so if you want to work on this part of the code, you may also have to create an `hie.yaml` files where `cabal` files reside.
 
+Frequently [implicit-hie](https://github.com/Avi-D-coder/implicit-hie) project helps by generating configuration for all projects by:
+```shell
+gen-hie > hie.yaml  # into the main HLS directory
+```
+that whould generate 1 file with a tree of configuration for all projects, that configuration may work as is or be tuned further with custom configuration.
+
 ### Manually testing your hacked HLS
 If you want to test HLS while hacking on it, follow the steps below.