Add example for using arrays as parameters

I had a problem with the dynamic creation of recurrence rules for more that one day a week. 

If you build an array with all days in which an event should occur, you cannot use the array directly as parameter to some methods, eg. Rule.weekly(2).day([:monday, :friday]). Instead, you have to splat the array - Rule.weekly(2).day(*[:monday, :friday]).

In my opinion, it would be better if the methods would accept arrays too. But at least the readme should contain an example for that (probably frequent) case. So, I have added some lines to the README.md.