Merge pull request GitbookIO#516 from jr0cket/patch-1

Add gitbook init command to "how to use" section

Author: SamyPesse

README.md

@@ -16,6 +16,11 @@ GitBook can be installed from **NPM** using:
 $ npm install gitbook -g
 ```
 
+Create the directories and files for a book from its [SUMMARY.md](https://github.com/GitbookIO/gitbook#book-format) file using 
+```
+$ gitbook init
+```
+
 You can serve a repository as a book using:
 
 ```