Clarify py-clob-client quickstart env setup

[UserSbondorf]

Add a note about configuring env variables for private key and funder before running write-enabled examples.

Mention that these examples send real requests to the CLOB API to encourage using test keys or small balances.

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Note

Adds a README note to configure env vars (private key/funder) before write-enabled examples and warns they hit production; recommend test keys or small balances.

• Docs (README):
  • Add guidance to set PRIVATE_KEY and optional FUNDER via environment variables before running write-enabled examples.
  • Note that examples send real requests to production and recommend using test keys or small balances.

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