docs: Add README.md for iota-json-rpc (#1986)

* docs: Add README.md for `iota-json-rpc`

* docs: Improve wording

* docs: Mention `IotaRpcModule` trait

crates/iota-json-rpc/README.md

@@ -0,0 +1,44 @@
+# iota-json-rpc
+
+The `iota-json-rpc` library crate provides a flexible framework for building JSON-RPC servers to expose IOTA APIs.
+It supports requests over HTTP and WebSocket, and is primarily to spawn a nested service in the `iota-indexer`
+or `iota-node` applications.
+
+It provides module implementations for various JSON-RPC server traits (
+e.g. `IndexerApiServer`, `GovernanceReadApiServer`, `MoveUtilsApiServer`) which are derived in the `iota-json-rpc-api`
+crate from JSON-RPC API definitions (e.g. `IndexerApi`, `GovernanceReadApi`, `MoveUtilsApi`).
+
+These modules are not automatically registered with the server and can be explicitly included based on the server's
+requirements. They are registered for `iota-node` in particular, using `AuthorityState` for the internal state.
+
+Modules can be registered with the server using the `register_module` method, which expects a module struct of
+type `IotaRpcModule` trait. The trait provides module information such as a list of supported JSON-RPC methods and OpenRPC documentation.
+
+## Usage
+
+The following example shows how to build, register RPC modules and start the server:
+
+```rust
+// Create a new `JsonRpcServerBuilder` with version information and a Prometheus registry
+let mut builder = JsonRpcServerBuilder::new(env!("CARGO_PKG_VERSION"), prometheus::default_registry());
+
+// Register the RPC modules that should be included in the server.
+let reader = IndexerReader::new(env!("DATABASE_CONNECTION_URL")) ?;
+builder.register_module(TransactionBuilderApi::new(reader.clone())) ?;
+builder.register_module(GovernanceReadApi::new(reader.clone())) ?;
+builder.register_module(CoinReadApi::new(reader.clone())) ?;
+
+// Define the default socket address for the server, parsing the IP address correctly.
+let default_socket_addr: SocketAddr = SocketAddr::new(
+    "0.0.0.0".parse().unwrap(),  // Correctly parse the IP address
+    9000,
+);
+
+// Specify a custom runtime for the server, if needed.
+let custom_runtime: Option<tokio::runtime::Handle> = None;
+
+// Start the server.
+builder
+.start(default_socket_addr, custom_runtime, Some(ServerType::Http))
+.await?;
+```