Update README.md for nonlinear ring example

Author: austin-hoover

examples/nonlinear_ring_4d/README.md

@@ -1,3 +1,9 @@
-In this example we measure the $x$-$x'$ and $y$-$y'$ distribution after every 20 turns in a linear periodic lattice + nonlinear rotationally symmetric kick [1]. MENT finds an initial 4D phase space distribution consistent with these projections.
+# 4D reconstruction in nonlinear ring
+
+<img src="saved/fig_action.png" width="600px">
+
+In this example we measure the $x-p_x$ and $y-p_y$ distribution after every 20 turns in a linear periodic lattice + nonlinear rotationally symmetric kick [1]. MENT finds an initial 4D phase space distribution consistent with these projections.
+
+[1] https://arxiv.org/abs/2405.05657
+
 
-[1] https://arxiv.org/abs/2405.05657