Roadmap for new version

[Progi1984]

• Migration to Github Actions from Travis CI
  • #634 : Added Github Actions #635
• Support for recents versions of PHP and remove support for PHP < 7.1
  • #634 : Add support PHP 7.4 to 8.0 & Remove support PHP < 7.1 #636
• Add PHPStan
  • #634 : Enable PHPStan #639
• Add PHPCSFixer
  • #634 : Enable PHPCSFixer  #637
  • #634 : Rules for PHPCSFixer #640
• Use Coveralls for replacing Scrutinizer
  • #634: Added link to Coveralls #648
• Remove all deprecated
  • #634 : Removed deprecated methods/classes #650
• Migration documentation to Markdown (@mkdocs)
  • #634 : Documentation (Mkdocs & Github Actions) #647
  • #634 : Changelog #649
• Migrate from PHPExcel to PhpSpreadsheet
  • #634 : Migrated use of PHPExcel to PhpSpreadsheet #652
• Define new exceptions for each error
  • #634 : Set specific exceptions #677
• Add declare strict_types for each file
  • #634 : Adding Strict Types #673
• Upgrade major version of PHPOffice\Common
  • #634 : Upgrade major version of PHPOffice\Common #679