README: add Radius (km, default 50) to Setup options

[caiosweet]

Motivation

• Document the Radius option in the Setup → Optional list so the README matches the available config options.
• Keep terminology consistent with const.py and config_flow.py by reflecting CONF_RADIUS and DEFAULT_RADIUS in the documentation.
• Ensure the documented options mirror the integration options presented in the options form UI.

Description

• Updated README.md to add an entry: 6. Radius (km, default 50) under the Setup → Optional list.
• Aligned wording with const.py and config_flow.py to reflect DEFAULT_RADIUS and CONF_RADIUS.
• This is a documentation-only change and does not modify runtime code.

Testing

• No automated tests were run for this documentation-only change.

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Codex Task