Added a short description of the plugin

plugins/geeknote/README.md

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+## ZSH-Geeknote
+
+[Geeknote](https://github.com/VitaliyRodnenko/geeknote) plugin for [oh-my-zsh framework](http://github.com/robbyrussell/oh-my-zsh).
+
+Plugins provides:
+
+- auto completion of commands and their options
+- alias `gn`
+
+You can find information how to install Geeknote and it's available commands on the [project website](http://www.geeknote.me/).
+
+Maintainer : Ján Koščo ([@s7anley](https://twitter.com/s7anley))