improve documentation on outputs

[samuelcolvin]

It's not that clear from the README how to use outputs.

It might help to include that "success" is the string, not the name of the variable set is result, also an example of usage.

I had to read the code to get this working.

[webknjaz]

Thanks, when I was adding the outputs, I mostly exposed them “just in case” with no testing or verification. I think it'd make more sense to guarantee that failure and success are booleans instead.

So there are three outputs: failure/success seem to be booleans and result is a string.