ACTION-286: Add editor\\\\\'s note to spec with list of missing sections.

Added a notice at the top of the document that says, \"This document is a preliminary snapshot of the work in progress draft of the EXI format specification. It is provided as a courtesy copy to permit other interested W3C members to have a chance to take a peek at its status in preparation for the joint working group meeting during 22-26 January 2007. Substantial changes and additions are expected before it becomes ready for any wider consumption. Several key sections that are missing in this document include XML Schema to EXI grammar binding, EXI format header specification and compression algorithm.\"