add README

Author: monkstone

processing_app/topics/filters/filters.md

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+filters4jruby_art
+==================
+A port of [Filters4Processing][] to propane
+A growing collection of pixel shaders ported to Processing to be used with the `filter()` function. Most of these shaders come from the excellent [Shadertoy](https://www.shadertoy.com) by Iñigo Quilez.
+
+[Filters4Processing]:https://github.com/SableRaf/Filters4Processing