Reproducible workflows intro

[willpearse]

...which, in my opinion, is the same as "how to use a Rakefile"

[DomBennett]

There's also packrat. I think perhaps we could work this by having a general 'Reproducible workflows intro' pointing at separate guides on specific software for creating them, such as rakefile and packrat.

[willpearse]

...there is also remake (https://github.com/richfitz/remake).

There are loads; I'm happy to do one on rake, but not the others :p