Fixing an indentation issue with the additionalProperties field in the Schemas of Entity and Relationship within the NGSI-LD OAS.

Update links within the README file and add a link to view the original NGSI-LD OAS in the Swagger Editor of ETSI Forge.

Create an initial fork repository to work on the OpenAPI Specification (OAS) for the NGSI-LD API specified by the cross-cutting Context Information Management (CIM) ETSI Industry Specification Group (ISG).