docs(readme): clarify signer compatibility (ethers v5 vs v6, viem)

[skyc1e]

Closes #256.

What changed

Adds a short Signer compatibility section to README.md that states explicitly which signers ClobClient accepts.

Why

The original example imports @ethersproject/wallet (ethers v5) without telling the reader that the library does not require ethers at all, and without flagging the ethers v6 mismatch.

src/signer.ts currently accepts:

interface EthersSigner {
    _signTypedData(domain, types, value): Promise<string>;
    getAddress(): Promise<string>;
}
export type ClobSigner = EthersSigner | WalletClient;

ethers v6 renamed _signTypedData → signTypedData, so an ethers v6 Wallet fails the isEthersTypedDataSigner check and, if it falls through to the viem path via isWalletClientSigner, its call signature does not match what signTypedDataWithSigner invokes. That is what issue #256 is describing.

The new section:

• Points new users to viem as the recommended path.
• Documents that ethers v5 (ethers@^5 or @ethersproject/wallet) works.
• States ethers v6 is not directly supported and explains why, with the recommended workaround.
• Notes that ethers is not a runtime dependency of this package.

Scope

• One file (README.md), additive section only.
• Does not touch existing Usage or Using viem WalletClient blocks.
• Does not overlap with PR Doc/add example signature type2 comment #257 (different section).

---

Note

Low Risk
Documentation-only change with no runtime or behavioral impact.

Overview
Adds a new Signer compatibility section to README.md that explicitly documents supported signer types for ClobClient (recommended viem WalletClient, ethers v5-compatible signers) and calls out that ethers v6 is not directly supported due to the _signTypedData rename.

Also clarifies that ethers is not a runtime dependency and only needs to be installed when using an ethers-based signer.

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