Add README for hello_world example

examples/hello_world/README.md

@@ -0,0 +1,20 @@
+# Hello World Example
+
+This example demonstrates a simple gRPC service using `roto`.
+
+## Running the server
+
+```bash
+cargo run --bin server
+```
+
+## Calling the service
+
+You can use `grpc_cli` to call the `HelloWorld` RPC:
+
+```bash
+grpc_cli call [::1]:50051 hello.HelloWorldService.HelloWorld 'name: "World"' \
+  --protofiles examples/hello_world/proto/hello.proto \
+  --proto_path examples/hello_world/proto \
+  --channel_creds_type insecure
+```