Adding instances in the ReadMe

As discussed [here](https://github.com/snoyberg/mono-traversable/issues/18), this is a draft for a simple guide to adding instances. I've included it in the Readme for now (and not the code documentation) to keep it simple.

What do you think?
Fixed small error
remove superfluous either section
Merge pull request #20 from jcristovao/master

Adding instances in the ReadMe