doc: add readme

README.md

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+# ioconnect-go
+
+* wrap c library [ioConnect](https://github.com/machinefi/ioConnect)
+* command line tool `didctl` for DID util 
+* did verifiable credential token service `srv-did-vc`
+
+[srv-did-vc docker registry](https://github.com/machinefi/ioconnect-go/pkgs/container/ioconnect-go)
+
+## run container
+
+```shell
+docker run --name srv-did-vc -d ghcr.io/machinefi/ioconnect-go:main
+```
+
+## exchange vc token by client DID
+
+```shell
+curl -X POST -d '{"clientID":"did:io:0x637e7a6d4ff1da58d17ede9785c21d7837bec429"}' http://127.0.0.1:9999/issue
+```
+
+## verify vc token and retrieve client DID
+
+```shell
+$ curl -X POST -d '{"token":"..."}' http://127.0.0.1:9999/verify
+```
+