Commented "supports es6" on isparta instrumenter (#97)

I'm not sure this is the best way to document that functionality, but it's a bit better than nothing.

README.md

@@ -135,6 +135,7 @@ var istanbul = require('gulp-istanbul');
 
 gulp.src('lib/**.js')
   .pipe(istanbul({
+    // supports es6
     instrumenter: isparta.Instrumenter
   }));
 ```