update readme regarding the flowVersion setting

Author: jseminck

README.md

@@ -40,6 +40,7 @@ You can also specify some settings that will be shared across all the plugin rul
       "createClass": "createReactClass", // Regex for Component Factory to use, default to "createReactClass"
       "pragma": "React",  // Pragma to use, default to "React"
       "version": "15.0" // React version, default to the latest React stable release
+      "flowVersion": "0.53" // Flow version
     },
     "propWrapperFunctions": [ "forbidExtraProps" ] // The names of any functions used to wrap the propTypes object, such as `forbidExtraProps`. If this isn't set, any propTypes wrapped in a function will be skipped.
   }