docs: add OpenRPC references

Author: zchee

docs/README.md

@@ -5,3 +5,15 @@
 - [JSON-RPC 2.0 Specification](https://www.jsonrpc.org/specification)
 - [golang.org/x/tools/internal/jsonrpc2](https://github.com/golang/tools/tree/master/internal/jsonrpc2)
 - [github.com/sourcegraph/jsonrpc2](https://github.com/sourcegraph/jsonrpc2)
+
+## OpenRPC
+
+The OpenRPC Specification defines a standard, programming language-agnostic interface description for JSON-RPC 2.0 APIs.
+
+- [open-rpc.org](https://open-rpc.org)
+- [OpenRPC Specification](https://spec.open-rpc.org/)
+- [OpenRPC GitHub organization](https://github.com/open-rpc)
+
+### examples
+
+- [etclabscore/ethereum-json-rpc-specification: A specification of JSON-RPC methods that an EVM-based blockchain client must implement.](https://github.com/etclabscore/ethereum-json-rpc-specification)