Add documentation for Serve feature

Author: Bobo1239

README.md

@@ -1,4 +1,4 @@
-# mdBook  
+# mdBook
 
 <table>
     <tr>
@@ -100,6 +100,10 @@ Here are the main commands you will want to run, for a more exhaustive explanati
 
     When you run this command, mdbook will watch your markdown files to rebuild the book on every change. This avoids having to come back to the terminal to type `mdbook build` over and over again.
 
+- `mdbook serve`
+
+    Does the same thing as `mdbook watch` but additionally serves the book at `http://localhost:3000` (port is changeable) and reloads the browser when a change occures.
+
 ### As a library
 
 Aside from the command line interface, this crate can also be used as a library. This means that you could integrate it in an existing project, like a web-app for example. Since the command line interface is just a wrapper around the library functionality, when you use this crate as a library you have full access to all the functionality of the command line interface with and easy to use API and more!