Add RelationshipType requirements for PNF Descriptor chapter

Proposed Change:

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5.1.10.6. Relationship Types

Requirement: R-64064

target: PNF

keyword: MUST

introduced:  Dublin

The PNFD provided by a PNF vendor MUST comply with the following Relationship Types as specified in ETSI NFV-SOL001 standard :

—tosca.datatypes.nfv.VirtualLinksTo

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