Created Implementation (markdown)

Implementation.md

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+# The pImpl idiom
+DirectXTK's implementation makes extensive use of the [pImpl idiom](http://en.wikipedia.org/wiki/Opaque_pointer). This keeps the public headers slim and minimizes inter-module dependencies.
+
+    // SpriteBatch.h public header
+    class SpriteBatch
+    {
+    public:
+        ...
+    
+    private:
+        // Private implementation.
+        class Impl;
+    
+        std::unique_ptr<Impl> pImpl;
+    
+        // Prevent copying.
+        SpriteBatch(SpriteBatch const&) DIRECTX_CTOR_DELETE
+        SpriteBatch& operator= (SpriteBatch const&) DIRECTX_CTOR_DELETE
+    };
+
+This also allows use to allocate the pImpl class internally using ``_aligned_malloc(x,16);`` so that we can use the DIrectXMath aligned XMVECTOR and XMMATRIX types directly in the implementation.
+
+# Compiler versions
+_DirectX Tool Kit_ supports a number of different versions of Visual C++. This requires some  macro tricks in the code.
+
+## Calling-conventions
+The ``std::function`` is used for callbacks as a general pattern so that client code can provide function pointers, lambdas, functors, etc. To support building with a mix of calling conventions, we need to annotate the ``std::function`` correctly. Due to a bug in VS 2010, this doesn't work correctly so we have to rely on defaults.
+
+    #ifndef DIRECTX_STD_CALLCONV
+    #if defined(_MSC_VER) && (_MSC_VER < 1700)
+    #define DIRECTX_STD_CALLCONV
+    #else
+    #define DIRECTX_STD_CALLCONV __cdecl
+    #endif
+    #endif
+
+_Note: ``std::function`` doesn't support using ``__vectorcall`` until VS 2015, so use of ``/Gv`` is difficult in VS 2013 or earlier._
+
+# Default constructors/assignment operators
+The C++11 standard includes a more efficient ``=default`` and ``=delete`` construct for dealing with default constructors and assignment operators. This is not supported until VS 2013, so we make use of the following macros:
+
+    #ifndef DIRECTX_CTOR_DEFAULT
+    #if defined(_MSC_VER) && (_MSC_VER < 1800)
+    #define DIRECTX_CTOR_DEFAULT {}
+    #define DIRECTX_CTOR_DELETE ;
+    #else
+    #define DIRECTX_CTOR_DEFAULT =default;
+    #define DIRECTX_CTOR_DELETE =delete;
+    #endif
+    #endif
+
+For example, to prevent copying we use:
+
+    // Prevent copying.
+    SpriteBatch(SpriteBatch const&) DIRECTX_CTOR_DELETE
+    SpriteBatch& operator= (SpriteBatch const&) DIRECTX_CTOR_DELETE
+
+_Note that use of ``=default`` and ``=delete`` can improve codegen for derived types._
+
+# Further Reading
+[Dual-use Coding Techniques for Games](http://blogs.msdn.com/b/chuckw/archive/2012/09/17/dual-use-coding-techniques-for-games.aspx)