Add Note Types for 508, 511 & 524

[kirkhess]

Re: lcnetdev/bibframe-ontology#122

We'd like to suggest that new note types be created and used for:

• 508 Credits: I - note - Note ; rdf:type rdf:resource="http://id.loc.gov/vocabulary/mnotetype/credits"

• 511 Performer: I - note - Note ; rdf:type rdf:resource="http://id.loc.gov/vocabulary/mnotetype/performer"
  This would match the other note patterns and stop having to interrogate if the literal begins with a specific string, support $6 and possibly $7 in the future.

• 524 Citation: I - note - Note ; rdf:type rdf:resource="http://id.loc.gov/vocabulary/mnotetype/prefCitation"
  This would allow support for $2, $3, and $6.