Mention .toJSON().

Technically the syntax for referring to an instance method is `PublicKeyCredential#toJSON()` but I think this is problematic. So I've avoided it.

Author: lgarron

README.md

@@ -4,6 +4,7 @@ As of March 2025, stable versions of all major browsers now support the followin
 
 - [`PublicKeyCredential.parseCreationOptionsFromJSON(…)`](https://developer.mozilla.org/en-US/docs/Web/API/PublicKeyCredential/parseCreationOptionsFromJSON_static)
 - [`PublicKeyCredential.parseRequestOptionsFromJSON(…)`](https://developer.mozilla.org/en-US/docs/Web/API/PublicKeyCredential/parseCreationOptionsFromJSON_static)
+- [`PublicKeyCredential` → `.toJSON()`](https://developer.mozilla.org/en-US/docs/Web/API/PublicKeyCredential)
 
 These should be used instead of `@github/webauthn-json`.