title: kotlinx-serialization class marked @Serializable does not have the .serializer() extension function
body: <p>I have the following data class</p>
  
  <pre><code>@Serializable
  data class Income(val id: String, 
                    val description: String, 
                    val amount: Int, 
                    val Time: Date, 
                    val userId: String)
  </code></pre>
  
  <p>now when I try to use the .serializer() function it says that the .serializer() is not defined for Income class so my project doesn't compile.</p>
  
  <pre><code>    val response = Json.stringify(Income.serializer(), Incomes)
      call.respond(HttpStatusCode.OK,response) 
  </code></pre>
  
  <p>I Looked twice on the documentation in the <a href="https://github.com/Kotlin/kotlinx.serialization#quick-example" rel="noreferrer">readme.md</a>. 
  Even watched the announcement video from <a href="https://www.youtube.com/watch?v=IhKTIFlNrC4" rel="noreferrer">KotlinConf</a>  </p>
  
  <p>Did anyone have the same problem. What am i doing wrong??</p>
  
  <p>Edit:</p>
  
  <p>I tried to just copy paste the samples from the readme.md and had the same problem. </p>
  
  <pre><code>import kotlinx.serialization.*
  import kotlinx.serialization.json.*
  
  @Serializable
  data class Data(val a: Int, val b: String = "42")
  
  fun main() {
      // Json also has .Default configuration which provides more reasonable settings,
      // but is subject to change in future versions
      val json = Json(JsonConfiguration.Stable)
      // serializing objects
      val jsonData = json.stringify(Data.serializer(), Data(42))
      // serializing lists
      val jsonList = json.stringify(Data.serializer().list, listOf(Data(42)))
      println(jsonData) // {"a": 42, "b": "42"}
      println(jsonList) // [{"a": 42, "b": "42"}]
  
      // parsing data back
      val obj = json.parse(Data.serializer(), """{"a":42}""") // b is optional since it has default value
      println(obj) // Data(a=42, b="42")
  }
  </code></pre>
  
  <p>This does not compile as well in my code.
  I'm currently using Kotlin 1.3..61 and 
  kotlinx-serialization-runtime 0.14.0</p>