add lights example readme

examples/lights/README.md

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+# Lights
+
+This example demonstrates powerful Porter-Duff composition in order to create a nice noisy light
+effect.
+
+**Use W and S keys** to adjust the level of "dust".
+
+The FPS is limited to 30, because the effect is a little expensive and my computer couldn't handle
+60 FPS. If you have a more powerful computer (which is quite likely), peek into the code and disable
+the limit.
+
+Credit for the panda art goes to [Ján Štrba](https://www.artstation.com/artist/janstrba).