Improve documentation in the Readme

[tim-kos]

I think the readme lacks quite a bit of documentation for all the available public methods. An example would be https://github.com/transloadit/node-sdk

It would be nice if the Readme also included where a template is used.

[kvz]

Talked some more about this and we agree that, without duplicating https://godoc.org/github.com/transloadit/go-sdk into this project's README.md, adding a near-copy-pastable example for adding a template would be a cool addition.