add a note in the Readme about difference between this lib and fluture

[adrian-gierakowski]

there is a "How is this Task different than Data.Task or Fun-Task" section however there is no mention of the difference between this library and fluture.

[jordalgo]

@adrian-gierakowski I'm not really working on this lib at the moment. Fluture is pretty much the same except more optimized and better tested. So I recommended using that one 👍