Adds Schema.org links

[HughP]

@tombaker For your consideration: This PR provides some links to schema.org entities which are relevant for DCMIType equivalents

[kcoyle]

I would avoid the Marc relators and types, or at least make them only one of the options. SRAP and cohorts like RIOXX are making use of COAR vocabularies. Their resource types are less numerous than MARK but they cover most of what someone might need without making them dig through hundreds of options. The COAR lists are also organized someone hierarchically rather than just alphabetic order, so sending someone there to find a term has a good likelihood of being satisfied.