Docs: Clarify use-cases and scope

[johnfosborneiii]

Per user feedback, some people have tried using other package manager instead of compiling from source, suggest updating the README with more clarity. Here is one suggestion (feel free to use or modify):

Original: "Build apk packages using declarative pipelines."
Modified: "Compile apk packages from source using declarative pipelines. This project is designed to build reproducible apk packages decoratively from source code using any native (make, PyBuilder, mvn, etc) tooling. Using other package managers (such as pip, npm) to generate an APK is out of scope and may result in errors.