RDF serialization for LEXIDMA

Hi all,
I updated the RDF serialization and added a section to the
DocBook specification for this. Unfortunately, I can't generate
the PDF at the moment as there is some issue with the PDF
generation (the build is failing). A couple of small notes:
I will have to revisit this when the renaming changes by
Michal (PR #36) are accepted
The properties `forLanguage` and `forPartOfSpeech` do not
describe what kind of values they take. I assumed a non-empty
string.
`placeholderMarker` does not describe any properties but has
`startIndex` and `endIndex` in the examples
The RDF ontology was implemented with OWL union classes but I
might change this to open classes before we finalize the spec.
Regards,
John

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John P. McCrae (he/him; #startsWithAName John (rhymes with  gone )

McCrae (rhymes with  hay ) /dÊÉn mÃkÉeÉ/) Assistant Professor - SFI Insight Centre for Data Analytics, Data Science Institute & Computer Science, University of Galway