Mention Either-type in the Readme

[h908714124]

Update the documentation to help people who are looking for an Either-type in Java.
Some poeple might not immediately know what a "Result" is.

[siy]

Good point. I'll do that. Meanwhile will keep the ticket open.

[siy]

I've added to README a few links to articles with more details and explanations. One of them explains in details why and how Either<L,R> was replaced with Result<T>.