Readme improvement - prepared action

[gitplaneta]

I've added this change to readme, because there's an unpleasant exception when using prepared statements for raw queries with no input arguments. Maybe the "values" should be an optional parameter?

For example, this causes problems when commented 'values':

    vertx.createHttpServer().requestHandler(req -> {

        JsonObject json = new JsonObject();
        json.putString("action", "prepared");
        json.putString("statement", "SELECT * FROM assetlink");
        //json.putArray("values", new JsonArray());

        vertx.eventBus().send("sql-executor", json, (Message<JsonObject> msg) -> {
            req.response().end(msg.body().encode());
        });
    }).listen(8080);

exception: "io.vertx.asyncsql.database.ConnectionHandler$$anonfun$prepared$1.apply(ConnectionHandler.scala:123) io.vertx.asyncsql.database.ConnectionHandler$$anonfun$prepared$1.apply(ConnectionHandler.scala:122) io.vertx.asyncsql.database.ConnectionHandler$$anonfun$sendWithPool$1.apply(ConnectionHandler.scala:114) io.vertx.asyncsql.database.ConnectionHandler$$anonfun$sendWithPool$1.apply(ConnectionHandler.scala:113) io.vertx.asyncsql.database.pool.AsyncConnectionPool$$anonfun$withConnection$2.apply(AsyncConnectionPool.scala:84) io.vertx.asyncsql.database.pool.AsyncConnectionPool$$anonfun$withConnection$2.apply(AsyncConnectionPool.scala:82) scala.util.Success$$anonfun$map$1.apply(Try.scala:206) scala.util.Try$.apply(Try.scala:161) scala.util.Success.map(Try.scala:206) scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:235) scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:235) scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32) org.vertx.scala.core.VertxExecutionContext$VertxExecutionContextImpl$$anonfun$execute$1.apply$mcV$sp(VertxExecutionContext.scala:49) org.vertx.scala.core.FunctionConverters$$anon$3.handle(FunctionConverters.scala:35) org.vertx.scala.core.FunctionConverters$$anon$3.handle(FunctionConverters.scala:34) org.vertx.java.core.impl.DefaultContext$2.run(DefaultContext.java:144) org.vertx.java.core.impl.DefaultContext$3.run(DefaultContext.java:175) io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:354) io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:353) io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:101) java.lang.Thread.run(Thread.java:745)"

[Narigo]

Thanks!