code: func (r *Resource) AddLinkCollection(rel Relation, l LinkCollection) {
  	n := r.Links[rel]
  	if n == nil {
  		//new link
  		r.Links[rel] = l
  		return
  	}
  
  	if nc, ok := n.(LinkCollection); ok {
  		r.Links[rel] = append(nc, l...)
  		return
  	}
  
  	if nl, ok := n.(Link); ok {
  		//prepend existing link to collection
  		r.Links[rel] = append(LinkCollection{nl}, l...)
  	}
  }
docs: // AddLinkCollection appends a LinkCollection to the resource.
  // l should be a LinkCollection