added description of new mixed serialization format ... and how it relates to old formats (sparse/dense)
added description of new mixed serialization format
... and how it relates to old formats (sparse/dense)

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa