Add documentation

I was tiring to invoke a function using the this lib. I had to jump between the wsk client code and this to figure out what to do here. For instance [here](https://github.com/apache/incubator-openwhisk-client-go/blob/7862108c109b7f1bbb47d66edb0a35d4a3c080a7/whisk/action.go#L287) it would have been really helpful to know that payload should be a `map[string]interface{}` to send data.