nyan: add README and deprecation warning (#7251)

plugins/nyan/README.md

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+# Nyan plugin
+
+This plugin adds a command to display [Nyan Cat](https://en.wikipedia.org/wiki/Nyan_Cat) right inside your terminal.
+
+**Plugin is deprecated**. Check [official repo](https://github.com/klange/nyancat) for more information.