Merge pull request #752 from jelen07/patch-1

Added Nette Tester to Testing section

README.md

 * [Kahlan](https://github.com/kahlan/kahlan) - Full stack Unit/BDD testing framework with built-in stub, mock and code-coverage support.
 * [Mink](https://mink.behat.org/en/latest/) - Web acceptance testing.
 * [Mockery](https://github.com/mockery/mockery) - A mock object library for testing.
+* [Nette Tester](https://github.com/nette/tester) - A productive and enjoyable parallel unit testing framework.
 * [ParaTest](https://github.com/paratestphp/paratest) - A parallel testing library for PHPUnit.
 * [Pest](https://pestphp.com/) - A testing framework with a focus on simplicity.
 * [Peridot](https://github.com/peridot-php/peridot) - An event-driven test framework.