update README with usage details

README.md

@@ -1,3 +1,38 @@
 # ResolutionScaler
 
 A simple script for managing resolution scaling in FNA.
+
+## Usage
+
+Call **ResolutionScaler.Init** with your display width and height, and your virtual width and height.
+
+```cs
+    Resolution.Init(
+        GraphicsDevice.PresentationParameters.BackBufferWidth,
+        GraphicsDevice.PresentationParameters.BackBufferHeight,
+            PLAY_AREA_WIDTH,
+            PLAY_AREA_HEIGHT
+    );
+```
+
+Then you can pass **ResolutionScaler.TransformMatrix** to FNA's *SpriteBatch* to properly transform your game's render target.
+
+```cs
+    SpriteBatch.Begin(
+        SpriteSortMode.Deferred,
+        null,
+        null,
+        null,
+        null,
+        null,
+        Resolution.TransformMatrix
+    );
+
+    SpriteBatch.Draw(
+        GameRenderTarget,
+        Vector2.Zero,
+        Color.White
+    );
+
+    SpriteBatch.End();
+```