rand-quote: add README

plugins/rand-quote/README.md

+# rand-quote plugin
+
+Displays a random quote taken from [quotationspage.com](http://www.quotationspage.com/random.php)
+
+Created by [Eduardo San Martin Morote, aka Posva](https://posva.github.io)
+
+## Usage
+
+Add the plugin to the plugins array in your zshrc file and restart zsh:
+
+```zsh
+plugins=(... rand-quote)
+```
+
+Then, run `quote` to get a new random quote.