document how to customize for more complex use cases

[agarwal]

Project.basic1 gets more attention than it was intended to. The set of functions it calls are the more useful contributions of solvuu-build. Document these other functions in the README, especially build_lib and build_app. Then users will realize the flexibility they really have, without substantial extra work.

[agarwal]

Similarly, the default solvuu.mk file isn't all that important, and even the generated _build/project.mk isn't critical. In README, explain that writing your own Makefile is reasonable.