Update README.md

Added link to post processing plugin

README.md

@@ -40,6 +40,7 @@ Please checkout [cura-build](https://github.com/Ultimaker/cura-build)
 Third party plugins
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 [Print time calculator](https://github.com/nallath/PrintCostCalculator)
+[Post processing plugin](https://github.com/nallath/PostProcessingPlugin)
 
 Making profiles for other printers
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