README: mention the supported use cases

[marten-seemann]

No description provided.

[Copilot]

Pull Request Overview

This pull request updates the README to reflect the supported use cases of connect-ip-go by detailing the proxy functionality for IP packets in HTTP/3.

• Modified the description to indicate connect-ip-go as an implementation of RFC 9484.
• Added a bullet list of supported use cases (Remote Access VPN and Site-to-Site VPN) with corresponding RFC references.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.98%. Comparing base (70172dd) to head (a3f26a6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #48   +/-   ##
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  Coverage   87.98%   87.98%           
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  Files           8        8           
  Lines         674      674           
=======================================
  Hits          593      593           
  Misses         62       62           
  Partials       19       19           

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