docs: add a tasteful star CTA near the top of the README

[Kayforkind]

Problem

Visitors decide in ~20 seconds whether to star. The README never asks.

Scope

• Add a single line near the top of the README, in the project's voice (plain, no hype), e.g.: "Find this useful? Click the star at the top of the page — it helps more than you think."

Acceptance criteria

• One line only, above the fold, no emoji spam, no begging.
• Matches the repo's plain-spoken tone.