add comment for stopping the simulation

PathPlanning/PotentialFieldPlanning/potential_field_planning.py

@@ -98,6 +98,7 @@ def potential_field_planning(sx, sy, gx, gy, ox, oy, reso, rr):
 
     if show_animation:
         draw_heatmap(pmap)
+        # for stopping simulation with the esc key.
         plt.gcf().canvas.mpl_connect('key_release_event',
                 lambda event: [exit(0) if event.key == 'escape' else None])
         plt.plot(ix, iy, "*k")