Add documentation for Kotlin notebooks serialization format

This commit includes a new document that explains the JSON serialization format used for rendering dataframes in the Kotlin notebooks plugin for IntelliJ IDEA. The format does not adhere to any formal schema such as Json Schema, but is intended for illustrative purposes.

Author: ermolenkodev

docs/serialization_format.md

@@ -0,0 +1,49 @@
+## Serialization format for the Kotlin notebooks plugin
+This document is an informal specification of the serialization format used for rendering Kotlin dataframes in the Kotlin notebooks plugin of IntelliJ IDEA.
+
+### Version 2.0.0
+### Top level json structure
+```json
+{
+    "$version": "2.0.0",
+    "metadata": {
+        "columns": [ string, ... ], // column names
+        "nrow": int,
+        "ncol": int
+    },
+    "kotlin_dataframe": [ Row, ... ]
+}
+```
+### Row
+```json
+{ 
+    "<value_column_name1>": string|Boolean|Double|Int|Float|Long|Short|Byte|list,,
+    "<value_column_name2>": string|Boolean|Double|Int|Float|Long|Short|Byte|list,,
+    ...
+    "<column_group_name1>": ColumnGroup,
+    "<column_group_name2>": ColumnGroup,
+    ...
+    "<frame_column_name1>": NestedFrame,
+    "<frame_column_name2>": NestedFrame
+}
+```
+### ColumnGroup
+```json
+{
+    "metadata": {
+        "kind": "ColumnGroup"
+    },
+    "data": Row
+}
+```
+### NestedFrame
+```json
+{
+    "metadata": {
+        "kind": "FrameColumn"
+        "nrow": int,
+        "ncol": int
+    },
+    "data": [ Row, ... ]
+}
+```