Require repositoryURL field but allow null value

[katucker]

The documentation for the repositoryURL field within the releases array of the code.json schema states the field is only required if the release is designated as Open Source. The JSON schema definition, however, specifies the field as required. As a result, the schema validator flags release entries with no repositoryURL field as invalid, regardless of the value of the corresponding usageType field.

I recommend modifying the documentation to state repositoryURL is required but can be assigned a null value if the corresponding release is not Open Source.

[DanielJDufour]

Good point @katucker . We'll look into this.