diff --git a/README.md b/README.md
index a5ba62c60..ed5eff44d 100644
--- a/README.md
+++ b/README.md
@@ -341,6 +341,7 @@ capitalization (`known_custom=custom` VS `sections=CUSTOM,...`) for all others r
 following mapping:
 
  - `known_standard_library` : `STANDARD_LIBRARY`
+ - `extra_standard_library` : `STANDARD_LIBRARY` # Like known standard library but appends instead of replacing
  - `known_future_library` : `FUTURE`
  - `known_first_party`: `FIRSTPARTY`
  - `known_third_party`: `THIRDPARTY`