Add doc about gitbook install

Author: SamyPesse

README.md

@@ -214,6 +214,8 @@ The platform [GitBook.io](https://www.gitbook.io/) is like an "Heroku for books"
 
 Plugins can be used to extend your book's functionality. Read [GitbookIO/plugin](https://github.com/GitbookIO/plugin) for more information about how to build a plugin for GitBook.
 
+Plugins needed to build a book can be installed using: `gitbook install ./`.
+
 ##### Official plugins:
 
 | Name | Description |