Releasing versions of extensions specification

[awoods]

The README is effectively a specification for how to define extensions. It is certainly conceivable that the README will continue to evolve. That said, it probably makes sense to:

• release versions of the extensions specification (i.e. the README), and
• add a field to extension definition headers that notes the version of the extensions specification to which an extension conforms, similar to the "Minimum OCFL Version" header field.

[bcail]

So, do we need to update the README with a "current version" field, and then add the "Extensions Specification Version" to the extension headers?

[awoods]

@bcail : Yes, both of those additions make sense to me.

[bcail]

#39