Add doc for plugins

docs/plugins.md

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+# Plugins
+
+Plugins are the best way to extend GitBook functionalities (ebook and website). There exist plugins to do a lot of things: bring math formulas display support, track visits using Google Analytic, etc.
+
+### How to find plugins?
+
+Plugins can be easily searched on [plugins.gitbook.com](https://plugins.gitbook.com).
+
+
+### How to install a plugin?
+
+Once you find a plugin that you want to install, you need to add it to your book.json:
+
+```
+{
+    "plugins": ["myPlugin", "anotherPlugin"]
+}
+```
+
+You can also specify a specific version using: "myPlugin@0.3.1". By default GitBook will resolve the latest version of the plugin compatbile with the current GitBook version.
+
+Plugins are automatically installed on [GitBook.com](https://www.gitbook.com). Locally, run `gitbook install` to install and prepare all plugins for your books.