Add screenshot to README.md

Author: emilk

README.md

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 This is a minimal CMake project that shows how to use [Rerun](https://github.com/rerun-io/rerun) in your code in conjunction with [Eigen](https://eigen.tuxfamily.org/) and [OpenCV](https://opencv.org/).
 
+<center>
+  <picture>
+    <img src="https://static.rerun.io/cpp-example-opencv-eigen/80ec7c698224eccb5ba1928136ba0a522d79b60a/full.png" alt="">
+    <source media="(max-width: 480px)" srcset="https://static.rerun.io/cpp-example-opencv-eigen/80ec7c698224eccb5ba1928136ba0a522d79b60a/480w.png">
+    <source media="(max-width: 768px)" srcset="https://static.rerun.io/cpp-example-opencv-eigen/80ec7c698224eccb5ba1928136ba0a522d79b60a/768w.png">
+    <source media="(max-width: 1024px)" srcset="https://static.rerun.io/cpp-example-opencv-eigen/80ec7c698224eccb5ba1928136ba0a522d79b60a/1024w.png">
+    <source media="(max-width: 1200px)" srcset="https://static.rerun.io/cpp-example-opencv-eigen/80ec7c698224eccb5ba1928136ba0a522d79b60a/1200w.png">
+  </picture>
+</center>
+
 ## Installing the Rerun viewer
 The Rerun C++ SDK works by connecting to an awaiting Rerun Viewer over TCP.