added README.md for json module

Author: Jolanrensen

dataframe-json/README.md

@@ -0,0 +1,14 @@
+## :dataframe-json
+
+This module, published as `dataframe-json`, contains all logic and tests for DataFrame to be able to work with
+JSON data sources; [reading](https://kotlin.github.io/dataframe/read.html#read-from-json)
+and [writing](https://kotlin.github.io/dataframe/write.html#writing-to-json).
+It's based on [Kotlinx Serialization](https://github.com/Kotlin/kotlinx.serialization).
+
+It also contains some logic specific to encoding dataframes as JSON objects with metadata for
+the [custom table component in Kotlin Notebook](https://kotlin.github.io/dataframe/usage-with-kotlin-notebook-plugin.html).
+See [serialization_format](../docs/serialization_format.md) for more information about the format.
+
+This module is optional but is included by default by the `dataframe` module, `dataframe-jupyter`,
+`dataframe-csv`, and `dataframe-excel`.
+If you want to use DataFrame without JSON support, you can exclude this module from the dependency.