docs: add readme

core/corehttp/gateway/README.md

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+# IPFS Gateway
+
+> IPFS Gateway HTTP handler.
+
+## Documentation
+
+* Go Documentation: https://pkg.go.dev/github.com/ipfs/kubo/core/corehttp/gateway
+
+## Example
+
+```go
+// Initialize your headers and apply the default headers.
+headers := map[string][]string{}
+gateway.AddAccessControlHeaders(headers)
+
+conf := gateway.Config{
+  Writable: false,
+  Headers:  headers,
+}
+
+// Initialize a NodeAPI interface for both an online and offline versions.
+// The offline version should not make any network request for missing content.
+ipfs := ...
+offlineIPFS := ...
+
+// Create http mux and setup gateway handler.
+mux := http.NewServeMux()
+gwHandler := gateway.NewHandler(conf, ipfs, offlineIPFS)
+mux.Handle("/ipfs/", gwHandler)
+mux.Handle("/ipns/", gwHandler)
+
+// Start the server on :8080 and voilá! You have an IPFS gateway running
+// in http://localhost:8080.
+_ = http.ListenAndServe(":8080", mux)
+```