document project-specific lang config

Author: kirawi

book/src/languages.md

@@ -12,3 +12,6 @@ name = "rust"
 auto-format = false
 ```
 
+# Project-level Configuration
+
+You can create a `.helix` directory either in the current working directory or in the same directory as `.git`. Then you can write a `languages.toml` file that will overwrite both global and default language settings.