add doc.go to each implementation

[matryer]

Add a doc.go file to each implementation with a small summary of what the package provides.

E.g.

// Package local provides a filesystem Location. Folders are containers, and 
// files are items.
package local

[piotrrojek]

The issue seems forgotten @Xercoy. I fixed that quickly, I hope you don't mind.

[Xercoy]

Discussed with @piotrrojek, did not forget, my contributions for this are more comprehensive. Will have a WIP up.