Change default value logic for stepCmd and cmdInput (see README).

README.md

@@ -125,7 +125,7 @@ Step definitions are JSON objects with the following keys:
   `'.'`, the project root directory)*.
 
 * `stepCmd` *(optional)*: the command to execute for this step. *(defaults to
-  `sh`)*
+  `true` unless `cmdInput` is given, in which case it defaults to `sh`)*
 
 * `cmdInput` *(optional)*: an array of string that will be concatenated with
   newlines separating each string and piped as input to the command for this