Add an instruction for running unit tests

Author: tomaszpieczykolan

README.md

@@ -43,3 +43,9 @@ mt -nologo -manifest Examples\UICatalog\UICatalog.exe.manifest -outputresource:.
 copy Examples\UICatalog\Info.plist .build\x86_64-unknown-windows-msvc\debug\
 .build\x86_64-unknown-windows-msvc\debug\UICatalog.exe
 ```
+
+## Running tests
+
+In order to run unit tests of Swift/Win32 you need to:
+1. Open `x64 Native Tools Command Prompt for VS 2019`.
+2. Run `swift test -Xswiftc -DENABLE_TESTING`.