Add manifest.rdf file if necessary

[jferard]

Part 1, 4.2.2 manifest.rdf:

The OpenDocument document contains a metadata manifest, as defined in Part 3 of this specification.

Part 3, 3.6 Metadata

All metadata files of a document or sub document shall be listed in a separate metadata manifest file, which has the file name “manifest.rdf”. This file enumerates metadata files and their relationships to other files in an OpenDocument package. See chapter 6.
The "manifest.rdf" file need not exist if a document or sub document does not contain any files that contain RDF metadata.

Part 3, 6 Metadata Manifest Files:

The metadata manifest file for a document (see 3.1) shall be stored in the root of the package. The metadata manifest file for a sub documents shall be stored in the sub document's directory.

Note: manifest.rdf is different from META-INF/manifest.xml. The former describes the document parts that contain RDF metadata, the latter is used to check the content of the document