added readme for unit tests

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.
+
+## Notes
+
+* Running tests in parallel is disabled because `Application` is a singleton. Do not change those settings.
+
+## Guidelines for Adding More Tests
+
+1. Do. Please. Add lots.
+2. Structure the tests by class. Name the test classes in the form of `ClassNameTests` and the file `ClassNameTests.cs`.
+3. The test functions themselves should have descriptive names like `TestBeginEnd`.
+4. IMPORTANT: Remember `Application` is a static class (singleton). You must clean up after your tests by calling `Application.Shutdown`.