Document how to extend a workflow

[jhclark]

Ducttape is built around the principle that workflows should be run, edited, and then re-run without losing work. However, certain workflow changes can cause work to be lost (that is tasks might be re-run unnecessarily). We should document which conditions cause this and which don't.

Conditions that might cause this include:

• Renaming a task
• Renaming a branch point
• Changing which branch comes first (changing the baseline branch)

Conditions that are legal:

• Adding branch points
• Adding branches to a branch point