docs: Add entry for oa token

[arnesetzer]

👋 I did some awesome work for the Pelias project and would love for everyone to have a look at it and provide feedback.

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Here's the reason for this change 🚀

imports.openaddresses.token was an undocumented config parameter. To encourage users to use their own oa token the parameter should be documented.

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Here's what actually got changed 👏

• README.md

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Here's how others can test the changes 👀

/

[missinglink]

Thanks, minor typo avaoid.

[arnesetzer]

Fixed typo 😄