v1.4.0

• Improve API documentation, update README and add examples.
  (#45 by @clue and #51 by @SimonFrings)

• Improve test suite and add .gitattributes to exclude dev files from exports.
  Prepare PHP 8 support, update to PHPUnit 9, run tests on PHP 7.4 and simplify test matrix.
  (#46, #47 and #50 by @SimonFrings)