docs(README): install snippets pin `= "0.3"` but current crate is 0.4.4

[skyc1e]

Summary

README.md tells users to depend on polymarket-client-sdk = "0.3" in three places, but the crate shipped 0.4.0 earlier this year and is now at 0.4.4 (released 2026-03-17). Because Cargo treats "0.3" as ^0.3, users copy-pasting from the README are silently pinned to the 0.3.x line and never resolve to any 0.4.x release.

Affected lines

• L51 — polymarket-client-sdk = "0.3" in the Getting started snippet
• L86 — polymarket-client-sdk = { version = "0.3", features = ["ws", "data"] }
• L352 — polymarket-client-sdk = { version = "0.3", features = ["ws"] }

Evidence

• Cargo.toml → version = "0.4.4"
• CHANGELOG.md lists [0.4.4] - 2026-03-17, [0.4.3] - 2026-02-25, with 0.4.0 landing between PR docs: update README with new features and examples #80 and now
• git log -S '= "0.3"' README.md shows the string has been unchanged since 0ee0979 (PR docs: update README with new features and examples #80, 2026-01-02) — before any 0.4.x release
• crates.io shows 0.4.4 as the current release

Impact

• New users following the Getting Started section get a version that is several minor releases behind, missing every API/fix added in 0.4.x (tracked in CHANGELOG.md).
• cargo add polymarket-client-sdk (also shown in the README) resolves to 0.4.4, so users get inconsistent behavior depending on whether they follow the TOML snippet or the cargo add line.

Suggested fix

Bump the three snippets to "0.4", and consider wiring the README version into release-plz so it bumps automatically with each release. I'm happy to send the README-only PR if you'd like.

Checklist

• Searched open + closed issues and PRs for README 0.3 / version bump / README 0.4 — no duplicates.