Update README.md

Be more explicit about requirements for Windows.  Add the need for Swift flags with CMake on WIndows.  Add explicit content for Windows and macOS and Linux.  This should clear up the confusion for people not familiar with the use of CMake for Swift.

Author: compnerd

Diff for: HelloWorld/README.md

@@ -19,12 +19,32 @@ and android).
 If `swiftc` is not in your path, you will need to add `-DCMAKE_Swift_COMPILER=`
 with the path to swiftc.
 
-```sh
-cmake -G Ninja -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo -DBUILD_TESTING=YES
-cd build
-ninja
-ninja test
+<details>
+  <summary>Linux or macOS</summary>
+
+
+```bash
+cmake -B build -D CMAKE_BUILD_TYPE=RelWithDebInfo -D BUILD_TESTING=YES -G Ninja -S .
+ninja -C build
+ninja -C build test
+```
+</details>
+
+<details>
+  <summary>Windows</summary>
+
+> **NOTE:** we must build with the Release configuration on Windows as the Swift runtime
+> in debug configuration is not distributed with the standard toolchain.  MSVCRT cannot
+> be used in different configurations in the same process, and will result in runtime
+> failures.
+
+```cmd
+set SWIFTFLAGS=-sdk %SDKROOT%
+cmake -B build -D CMAKE_BUILD_TYPE=Release -D BUILD_TESTING=YES -G Ninja -S .
+ninja -C build
+ninja -C build test
 ```
+</details>
 
 This invocation builds the project in release mode with debug information.  This
 enables optimized builds with debug information.  Additionally, the standard