func (resource *DocumentManifest) MarshalJSON() ([]byte, error) {
	resource.ResourceType = "DocumentManifest"
	return json.Marshal(*resource)
}
Custom marshaller to add the resourceType property, as required by the specification
package models
import (
	"encoding/json"
	"errors"
	"fmt"
)
type DocumentManifest struct {
	DomainResource   `bson:",inline"`
	MasterIdentifier *Identifier                        `bson:"masterIdentifier,omitempty" json:"masterIdentifier,omitempty"`
	Identifier       []Identifier                       `bson:"identifier,omitempty" json:"identifier,omitempty"`
	Subject          *Reference                         `bson:"subject,omitempty" json:"subject,omitempty"`
	Recipient        []Reference                        `bson:"recipient,omitempty" json:"recipient,omitempty"`
	Type             *CodeableConcept                   `bson:"type,omitempty" json:"type,omitempty"`
	Author           []Reference                        `bson:"author,omitempty" json:"author,omitempty"`
	Created          *FHIRDateTime                      `bson:"created,omitempty" json:"created,omitempty"`
	Source           string                             `bson:"source,omitempty" json:"source,omitempty"`
	Status           string                             `bson:"status,omitempty" json:"status,omitempty"`
	Description      string                             `bson:"description,omitempty" json:"description,omitempty"`
	Content          []DocumentManifestContentComponent `bson:"content,omitempty" json:"content,omitempty"`
	Related          []DocumentManifestRelatedComponent `bson:"related,omitempty" json:"related,omitempty"`
}