Add file extension for v2.1

Documentation/README.md

@@ -29,7 +29,8 @@ Globally Unique ID in the IFC format. This format is used only when referring to
 * This document describes the BCF format that is used to exchange topics, such as, issues, scenes, etc. between different BIM software.
 
 ### BCF file structure
-A BCF file is a zip containing one folder for each topic. The root of the BCF zip contains the following files.
+A BCF file is a zip containing one folder for each topic with its file extension "bcfzip" for BCFv1.0 and BCFv2.0. The file extension as the version number "bcfv2.1" is introduced since BCFv2.1.
+The root of the BCF zip contains the following files.
 
 * project.bcfp (optional)
     - An XML file referencing the extension.xsd to a project. The schema for this file is project.xsd.