Add note to provide clarification about version numbers

Author: rgaiacs

docs/format_description.rst

@@ -4,6 +4,10 @@
 The Notebook file format
 ========================
 
+.. note::
+
+    The version number for the Jupyter Notebook format is independent of the version number for the reference implementation library.
+
 The official Jupyter Notebook format is defined with
 `this JSON schema <https://github.com/jupyter/nbformat/blob/master/nbformat/v4/nbformat.v4.schema.json>`_,
 which is used by Jupyter tools to validate notebooks.