Discussion on what should be a target

[wlandau]

Seems to come up a lot, including here. The chapter on plans should discuss the appropriate use of targets, and it should expound the virtues of functions too.

With enough discussion, we can separate section on large plans into its own chapter. Dynamic branching can then go in another chapter of its own.

[wlandau]

Should probably talk about what objects should be saved. Object size and type matter. ropensci/drake#383.

[wlandau]

Also avoid things with session-dependent pointers: xgboost objects and Keras models (unless you set format = "keras").

[wlandau]

Fixed in e782082.