text: Title: Feature request: support readOnlyRecommended
  Body: Hi,
  
  While not in the specifications, Excel (online) has a "Protect Workbook" feature that changes the default open behaviour for a spreadsheet to readonly.
  This is done by adding a readOnlyRecommended attribute to a fileSharing node on the workbook root node like so
  `<fileSharing readOnlyRecommended="1"/>`
  (in excel just after the fileVersion node)
  
  I've done this locally by adding
  `self._xml_empty_tag('fileSharing', [('readOnlyRecommended', "1")])`
  to Workbook._assemble_xml_file()
  