Update CONTRIBUTING.md

Author: vors

CONTRIBUTING.md

@@ -5,3 +5,7 @@ There are two permanent brances: **master** and **dev**.
 **Master** always points to the latest release tag. 
 All development is done in **dev** branch, 
 please, use it for pull requests.
+
+If you are fixing grammar (syntax highlighting), please, add affected samples to [test-file.ps1](/tests/samples/test-file.ps1). 
+We use it in automated tests that compare grammar against PowerShell tokenizer.
+It also serves as a place for humans to stair into all exotic language constructions.