Improve batch call example and wrapper comments (#214)

README.md

@@ -394,6 +394,20 @@ You can use:
 let bmiIndex = await client.bmi(120.5, 12.0)
 ```
 
+Or you can use batch call to send multiple request at once to the server.
+
+```Nim
+let batch = client.prepareBatch()
+batch.bmi(120.5, 12.0)
+batch.bmi(120.5, 13.0)
+batch.bmi(120.5, 14.0)
+let res = await batch.send()
+
+# But you need to manually process the response e.g. decode from JSON to
+# your expected type because you can mix various rpc method call in one batch
+# with various return type.
+```
+
 This allows you to leverage Nim's static type checking whilst also aiding readability and providing a unified location to declare client side RPC definitions.
 
 ## Working with client transports

json_rpc/private/client_handler_wrapper.nim

@@ -67,6 +67,13 @@ proc createRpcFromSig*(clientType, rpcDecl: NimNode, alias = NimNode(nil)): NimN
   ##     reqParams.positional.add encode(JrpcConv, paramB).JsonString
   ##     let res = await client.call("rpcApi", reqParams)
   ##     result = decode(JrpcConv, res.string, typeof RetType)
+  ##
+  ## 2nd version to handle batch request after calling client.prepareBatch()
+  ## proc rpcApi(batch: RpcBatchCallRef; paramA: TypeA; paramB: TypeB) =
+  ##   var reqParams = RequestParamsTx(kind: rpPositional)
+  ##   reqParams.positional.add encode(JrpcConv, paramA).JsonString
+  ##   reqParams.positional.add encode(JrpcConv, paramB).JsonString
+  ##   batch.batch.add RpcBatchItem(meth: "rpcApi", params: reqParams)
 
   # Each input parameter in the rpc signature is converted
   # to json using JrpcConv.encode.