Add how to set unsupported headers to readme

[jagaapple]

🌱 Feature Request

Is your feature request related to a problem? Please describe.

Next.js supports to set any headers, but some developers don't know about it.

Describe the solution you'd like

Readme should be written how to set unsupported headers in next-secure-headers using the standard way in Next.js.

Describe alternatives you've considered

None.

Documentation, Adoption, Migration Strategy

None.

Additional context

This will be released in 2.3.0.

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• [x] I've tried to find similar issues and pull requests
• [x] I would like to work on this feature 💪🏻