Missing OCF context

[mattgarrish]

In merging the specs, we dropped the purpose and scope sections, but I noticed that we lost some of the OCF explanation in doing so. The OCF section now begins by immediately jumping into the abstract container.

It feels like we should restore this text as an introduction:

OCF is the required container technology for EPUB Publications. OCF may play a role in the following workflows:

• During the preparation steps in producing an electronic Publication, OCF may be used as the container format when exchanging in-progress Publications between different individuals and/or different organizations.

• When providing an electronic Publication from publisher or conversion house to the distribution or sales channel, OCF is the recommended container format to be used as the transport format.

• When delivering the final Publication to an EPUB Reading System or User, OCF is the required format for the container that holds all of the assets that make up the Publication.

The OCF specification defines the rules for structuring the file collection in the abstract: the "abstract container". It also defines the rules for the representation of this abstract container within a ZIP archive: the "physical container". The rules for ZIP physical containers build upon the ZIP technologies used by [ODF]. OCF also defines a standard method for obfuscating embedded fonts for those EPUB Publications that require this functionality.