feat: add screenshot to readme

Author: Segergren

README.md

@@ -124,6 +124,22 @@ using var image = ImageSource.FromFile("logo.png");
 using var media = new MediaSource("Video", "video.mp4").SetLooping(true);
 ```
 
+## Screenshots
+
+```csharp
+// Full source screenshot (returns BGRA pixels, width, height — or null)
+var screenshot = source.TakeScreenshot();
+if (screenshot != null)
+{
+    using var bmp = new Bitmap((int)screenshot.Width, (int)screenshot.Height, (int)(screenshot.Width * 4),
+        PixelFormat.Format32bppArgb, Marshal.UnsafeAddrOfPinnedArrayElement(screenshot.Pixels, 0));
+    bmp.Save("screenshot.jpg", ImageFormat.Jpeg);
+}
+
+// Cropped screenshot — only transfers the crop region from GPU
+var cropped = source.TakeScreenshot(cropX: 960, cropY: 200, cropWidth: 640, cropHeight: 160);
+```
+
 ## Encoders
 
 ```csharp