Clarify story for modifying a task from a preset

@ecraig12345 2019-02-21 00:51:58发表于 microsoft/just Area: DocumentationNeeds: Design :thought_balloon:Package: just-scripts

Suppose I want to mostly use the webapp preset as-is, but I want to modify the 'build' task (maybe to build scss or codepen examples before ts). How should I do that?

Related question: how do I know the names of the existing tasks provided by a preset?