add extensions specification version (#39)

* add extensions specification version

* update version location

* update header name

README.md

@@ -1,5 +1,7 @@
 # OCFL Community Extensions
 
+**Version**: 1.0
+
 This repository contains community extensions to the [OCFL Specification and Implementation Notes](https://ocfl.io/). Extensions are a means of adding new functionality and documenting standards outside of the main OCFL specification process. For example, storage layout extensions define how OCFL object IDs are mapped to OCFL object root directories within an OCFL storage root. This mapping is outside of the scope of the OCFL specification, but is valuable information to capture so that repositories are self-describing and easily accessible using generic OCFL tooling.
 
 This is a community driven repository. Community members are encouraged to contribute by submitting new extensions and reviewing others' submissions. For more details, see the [review/merge policy](#review--merge-policy) below.
@@ -82,6 +84,7 @@ Extension definitions MUST contain a header section that defines the following f
 * **Extension Name**: The extension's unique *Registered Name*
 * **Authors**: The names of the individuals who authored the extension
 * **Minimum OCFL Version**: The minimum OCFL version that the extension requires, eg. *1.0*
+* **OCFL Community Extensions Version**: The version of the OCFL Extensions Specification that the extension conforms to, eg. *1.0*
 * **Obsoletes**: The *Registered Name* of the extension that this extension obsoletes, or *n/a*
 * **Obsoleted by**: The *Registered Name* of the extension that obsoletes this extension, or *n/a*