Update README to include Discussion about the flow of files through the Rodeo

[jeremyf]

Within the context of the DerivativeRodeo we have conceptual data structures associated with the file set.

• Chute: What are the files we’re bringing to the rodeo
• Agenda: What files are the rodeo responsible for making
• RoundUp: What files do we actually have when all is done; what we will associate with the corresponding FileSet.

The DerivativeRodeo is responsible for making the files required by the Agenda that are not in the Chute (e.g. Agenda - Chute). The RoundUp is the Agenda ∪ Chute;

Related to:

• https://github.com/scientist-softserv/utk-hyku/issues/343
• https://github.com/scientist-softserv/adventist_knapsack/issues/421
• scientist-softserv/adventist-dl#3

[jeremyf]

This has been done through numerous commits.