ProcessKiller: add a readme

Author: JohnTheGr8

Plugins/Flow.Launcher.Plugin.ProcessKiller/Readme.md

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+Process Killer Plugin
+=====================
+
+A Flow-Launcher plugin that allows you to kill running processes.
+
+Based on Wox.Plugin.ProcessKiller, originally created by [cxfksword](https://github.com/cxfksword/Wox.Plugin.ProcessKiller) and improved by [theClueless](https://github.com/theClueless/Wox.Plugins/tree/master/Wox.Plugin.ProcessKiller).
+
+![image](https://user-images.githubusercontent.com/697917/86478286-b4697700-bd52-11ea-875a-b3b90fa34bf2.png)