add known issue about libreoffice issue for spreadsheet generation

related to #186

PR Goal

[ ] If relevant, manual tests have been done to ensure the stability of the whole application and that the involved feature works
[ ] Automated tests covering the feature or the fix, have been written, deemed irrelevant (give the reason), or an issue has been created to implement the test (give the link)
[ ] The PR or the original issue contains a short summary of the implemented solution.