README.md: valid subsystems are definded in lib/rules/subsystem.js (nodejs#38)

Author: cclauss

README.md

@@ -32,6 +32,7 @@ $ core-validate-commit --list
       title-format enforce commit title format
       title-length enforce max length of commit title
 ```
+Valid subsystems are defined in [lib/rules/subsystem.js](./lib/rules/subsystem.js).
 
 ## Test