docs(readme): add per-market fee rate section

[skyc1e]

Refs #326.

What changed

Adds a short Fee rates (per market) section to README.md showing how to query the live fee rate for a given token via the existing get_fee_rate_bps(token_id) method.

Why

The library already calls GET /fee-rate?token_id=... under the hood when creating orders, but the README never mentions the method or the fact that fees are market-specific. Issue #326 reports that the public fee docs imply a single static rate per category, while /fee-rate actually returns different values per market (e.g. 0 for some NHL markets, 1000 bps for NBA/MLB).

This PR stays strictly within what the code and endpoint already do:

• Documents the public get_fee_rate_bps method.
• States the unit (basis points) and shows how to call it.
• Notes that the client will raise if a user-supplied fee_rate_bps conflicts with the resolved market rate — this matches the behavior in __resolve_fee_rate.
• Points readers to CalculatorHelper.sol for the authoritative on-chain fee calculation.

Scope

• One file (README.md), additive section only.
• No code changes.
• Deliberately does not assert a specific fee formula or category-level rate — those questions in Fee documentation contradicts CLOB API fee-rate endpoint for Sports markets #326 are for maintainers to answer on the public docs site.

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Note

Low Risk
Documentation-only change with no runtime or API behavior modifications.

Overview
Adds a new “Fee rates (per market)” section to README.md documenting that fees vary by market and can change over time, and showing how to query the live rate via client.get_fee_rate_bps(token_id).

Clarifies units (basis points), notes order creation will validate any user-supplied fee_rate_bps against the market’s resolved rate, and links to the on-chain fee calculation source (CalculatorHelper.sol).

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