Merge pull request #298 from Kotlin/spark-hint

Hint about Kotlin Spark API in I/O docs

Author: Jolanrensen

docs/StardustDocs/topics/io.md

@@ -2,3 +2,8 @@
 
 When you work with data, you have to [read](read.md) it from disk or from remote URLs and [write](write.md) it on disk.
 This section describes how to do it. For now, CSV, TSV, JSON, XLS, XLSX, Apache Arrow formats are supported.
+
+Working with [Apache Spark](https://spark.apache.org/) directly from Kotlin DataFrame is not possible at the time of 
+writing. However, there is a [Kotlin Spark API](https://github.com/Kotlin/kotlin-spark-api) which we can recommend in
+the meantime. If you do want to work with data from Spark in Kotlin DataFrame, we recommend exporting your data to CSV or
+the Apache Arrow format and then reading it back into a [DataFrame](DataFrame.md).