code: func (s *PullRequestsService) RequestReviewers(ctx context.Context, owner, repo string, number int, reviewers ReviewersRequest) (*PullRequest, *Response, error) {
  	u := fmt.Sprintf("repos/%s/%s/pulls/%d/requested_reviewers", owner, repo, number)
  	req, err := s.client.NewRequest("POST", u, &reviewers)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	r := new(PullRequest)
  	resp, err := s.client.Do(ctx, req, r)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return r, resp, nil
  }
docs: // RequestReviewers creates a review request for the provided reviewers for the specified pull request.
  //
  // GitHub API docs: https://developer.github.com/v3/pulls/review_requests/#create-a-review-request