Merge pull request #649 from tig/unittest_readme

Unittest readme

UnitTests/README.md

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+# Automated Unit Tests
+
+`Terminal.Gui` uses [xunit](https://xunit.net/) for automated unit tests run automatically with Github Actions.
+
+See the [Testing wiki](https://github.com/migueldeicaza/gui.cs/wiki/Testing) for details on how to add more tests.