Add demo screenshot

README.md

-# Nullifying Algorithms for Antenna Arrays
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+# Nullifying Algorithms for Antenna Arrays
+
+This project is a supplementary simulation tool for different algorithms, developed for the paper [Practical null forming in millimeter-wave networks](https://www.ideals.illinois.edu/handle/2142/108711).
+
+![Demo screenshot of the visualizer](/assets/screenshot.png)
+*Demo of the visualizer for a null at 70 degrees*