Added an article about row-oriented operations with purrr

[chsafouane]

Issue 610: row-oriented doc

[hadley]

Fixes #610

[lionel-]

Is this good to merge for the point release @hadley?

[lionel-]

@chsafouane, since the examples are taken from https://github.com/jennybc/row-oriented-workflows, could you include Jenny as co-author please?

[chsafouane]

I've added Jenny as a co-author. I think we might add a link to her repo and cite it in the vignette as I've only turned her repo to a vignette (little formatting and changed some of the examples)

[lionel-]

Agreed, a link and a mention would be great.

[chsafouane]

I've just added a reference to Jenny's repo

[hadley]

I now think this is out of scope for purrrr, since it falls an the documentation space that lies in between multiple packages. We don't currently have a good home for such docs, but @batpigandme is working on it, and this work is likely to make a future appearance elsewhere.