Add deprecation notice

Author: fletchto99

README.md

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+⚠️ ⚠️ ⚠️
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+Webauthn-json has been sunset. Now that all major browsers support webauthn we recommend invoking the native APIs
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+⚠️ ⚠️ ⚠️
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 # `@github/webauthn-json`
 
 `@github/webauthn-json` is a client-side Javascript library that serves as convenience wrapper for the the [WebAuthn API](https://www.w3.org/TR/webauthn/) by encoding binary data using [base64url](https://w3c.github.io/webauthn/#sctn-dependencies) (also known as "websafe" or "urlsafe" base64).