Document ESM extension requirement

Author: transphorm

common/README.md

@@ -1,3 +1,7 @@
 # OpenPassport Commons
 
 Constants and utils used in multiple OpenPassport subdirectories.
+
+**Note**: Source files use explicit `.js` extensions in internal imports. TypeScript's
+`nodenext` module setting requires file extensions for ESM, so dropping them
+would cause type-check failures.