Relationship between molecule and Compose UI

Considering the common confusion around Compose / Compose UI it might be helpful to add a small section to the README explaining the relationship between molecule and Compose UI:

- You don't need to use Compose UI in order to use molecule
- If your app is fully Compose UI you don't necessarily need to use molecule
- If you don't need the output to be a `StateFlow` you don't need molecule and can call "presenter" functions (from Compose UI's composition) that return a `State`
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Start with the README and review the existing explanations of Molecule and Compose UI. Add a small section covering when Molecule is and is not needed, including the StateFlow distinction and the possible pros, cons, and added complexity. Done means the relationship and decision points are clear to readers.

Written by the indexing model from the issue text.