Update README.md

CMake 3.15 is the actual current release, the git version of CMake is no longer needed.  Use `-B` to avoid hardcoding the paths.  This makes it possible to build without a reference to the checkout location.  Update to build in release mode.

Author: compnerd

README.md

@@ -4,15 +4,11 @@ This is a thin wrapper over the Win32 APIs for graphics on Windows to make it
 easier to write Windows applications in Swift.
 
 This requires the latest (git) version of Swift (alternatively, you can use the
-nightlies from https://dev.azure.com/compnerd/windows-swift).  Additionally, you
-need the latest (git) version of CMake (from http://github.com/KitWare/cmake).
-
-Assuming you are building in `S:`, you can build with:
+nightlies from https://dev.azure.com/compnerd/windows-swift).
 
 ```cmd
-cmake -G Ninja S:/swift-win32
-ninja SwiftWin32
-ninja HelloSwift
-.\HelloSwift
+cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release .
+ninja -C build SwiftWin32 HelloSwift
+.\build\HelloSwift
 ```