code: func (s *AppsService) AddRepository(ctx context.Context, instID, repoID int64) (*Repository, *Response, error) {
  	u := fmt.Sprintf("user/installations/%v/repositories/%v", instID, repoID)
  	req, err := s.client.NewRequest("PUT", u, nil)
  	if err != nil {
  		return nil, nil, err
  	}
  	req.Header.Set("Accept", mediaTypeIntegrationPreview)
  
  	r := new(Repository)
  	resp, err := s.client.Do(ctx, req, r)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return r, resp, nil
  }
docs: // AddRepository adds a single repository to an installation.
  //
  // GitHub API docs: https://developer.github.com/v3/apps/installations/#add-repository-to-installation