Add 'Optional Extension Initializer' (#36)

* Add 'Optional Extension Initializer'

Resolves: #34

* code review

* typo

* code review

* code review

* code review

Co-authored-by: Andrew Woods <awoods@duraspace.org>

README.md

@@ -14,20 +14,23 @@ To use OCFL extensions you first need an OCFL client that supports the desired e
 
 Reference the OCFL specification's description of [object extensions](https://ocfl.io/1.0/spec/#object-extensions) and [storage root extensions](https://ocfl.io/1.0/spec/#storage-root-extensions).
 
-A *root extension directory* refers to the directory named `extensions` that is located in either the storage root or an object root. An *extension directory* is an extension specific directory that is the child of a root extension directory and named using the extension's *Registered Name*. For example, `extensions/0000-example-extension` is the extension directory for the extension [0000-example-extension](docs/0000-example-extension.md).
+The OCFL storage root MAY contain a copy of an extension's specification.
 
 Each extension specification details how it should be implemented, but there are a few rules that apply to every extension.
 
-The OCFL storage root MAY contain a copy of an extension's specification.
+A *root extension directory* refers to the directory named `extensions` that is located in either the storage root or an object root. An *extension directory* is an extension specific directory that is the child of a root extension directory and MUST be named using the extension's *Registered Name*, or `initial` (see [Optional Initial Extension](#optional-initial-extension)). For example, `extensions/0000-example-extension` is the extension directory for the extension [0000-example-extension](docs/0000-example-extension.md).
 
 ### Configuration Files
 
 An extension's parameters are serialized as a JSON object and written to a configuration file named `config.json` within the extension's extension directory.
 
+If an extension includes a configuration file, one of the properties in that file MUST be `extensionName`, where the value is the *Registered Name* of the extension.
+
 For example, the extension [0000-example-extension](docs/0000-example-extension.md) could be parameterized as follows:
 
 ```json
-{ 
+{
+  "extensionName": "0000-example-extension",
   "firstExampleParameter": 12, 
   "secondExampleParameter": "Hello", 
   "thirdExampleParameter": "Green" 
@@ -45,6 +48,17 @@ It is conceivable that some extensions may not be compatible with other extensio
 
 Because OCFL clients are not required to implement any or all extensions, it is also possible that a client may encounter an extension that it does not implement. In these cases, it is up to the client to decide how to proceed. A client may fail on unsupported extensions, or it may choose to ignore the extensions and carry on.
 
+### Optional Initial Extension
+
+A _root extension directory_ MAY optionally contain an _initial_ extension that, if it exists, SHOULD be applied before all other extensions in the directory.
+An _initial extension_ is identified by the extension directory name "initial".
+
+An _initial extension_ could be used to address some of the [undefined behaviors](#undefined-behavior), define how extensions are applied, and answer questions such as:
+
+   - Is an extension deactivated, only applying to earlier versions of the object?
+   - Should extensions be applied in a specific order?
+   - Does one extension depend on another?
+
 ## Specifying Community Extensions
 
 ### Layout