Recommend declaring flwrcrate as a project dependency

[faizollah]

The README told users to run pip install flwrcrate directly. That installs into whatever environment happens to be active, and the dependency is lost when the project is set up in a new environment or on another machine.

Document adding flwrcrate>=0.4.0 to the Flower app's pyproject.toml dependencies as the primary path, keeping the direct pip install as a clearly-marked quick-try option (with the same-environment caveat moved there, since that is the only path where the mistake is possible).

Closes #12