add a README

Author: marten-seemann

README.md

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+# Fork of encoding/json
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+This is a fork of the `encoding/json` package that allows use of the `encoding/json/v2` package without requiring the `GOEXPERIMENT=jsonv2` environment variable. The code is based on the Go 1.25.1 release.
+
+This repository will be archived once the standard library is updated to allow use of the `encoding/json/v2` package without build constraints.
+
+No changes will be made to the codebase except for updating it to the latest Go release.