Improve wb_add_data() error message when no worksheet

[olivroy]

Before this, I got

wb_workbook()$add_data(x = mtcars)
Erreur dans wb$is_chartsheet[[sheet]] : 
  tentative de sélection de moins d'un élément dans integerOneIndex

Also added an example of adding a URL hyperlink (I didn't know how to do this, so I documented it!) + added a NEWS bullet + re-rendered README since I forgot to do so in #936

[JanMarvin]

Thanks, looking good!