Add an instruction for running unit tests

Author: tomaszpieczykolan

README.md

@@ -43,3 +43,16 @@ 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` as administrator.
+2. `cd` to the location of Swift/Win32 on your local computer.
+3. Run `swift test -Xswiftc -DENABLE_TESTING`.
+
+### Required software for running tests
+
+| Component | Visual Studio ID |
+| --- | --- |
+| Git for Windows | Microsoft.VisualStudio.Component.Git |