updated README.md

update_component_index.py

@@ -6,6 +6,9 @@ table in README.md with the actual contents of the library.
 Reads the following fields from each symbol:
   Value, MPN, Digikey_PN, Manufacturer, Footprint, ki_description (or Description)
 
+Symbol Library column = which .kicad_sym file the symbol lives in.
+Used to locate a symbol quickly in KiCad Symbol Editor.
+
 Run locally:   python3 .github/scripts/update_component_index.py
 Run in CI:     triggered automatically on push to symbols/
 """