Add inline documentation

[royhadad]

When using javascript arrays, we get the following documentation for each method:

When using this library, we don't get any docs:

Proposed feature: Add short documentation for the methods in the package

[elliotchance]

Yes, you're right. We have docs for all functions, such as: https://github.com/elliotchance/pie/blob/master/v2/map.go

But they need to be copied to the chained versions: https://github.com/elliotchance/pie/blob/master/v2/of.go

Happy to accept a PR for this... 😄