Added skeleton for the user manual

UserManual.md

+Overview
+--------
+
+Usage Examples
+--------------
+
+Command Line Options
+--------------------
+
+Common Problems
+---------------
+
+Embedding
+---------
+
+Other Resources
+---------------
+- A presentation made by Arnout Vandecappelle at FOSDEM 2011 on 2011/02/06
+  in Brussels, Belgium, called
+  "Creating secure web based user interfaces for Embedded Devices"
+  ([pdf](http://mind.be/content/110206_Web-ui.pdf) |
+   [odp](http://mind.be/content/110206_Web-ui.odp))
+- A Linux Journal article by Michel J.Hammel, 2010-04-01, called
+  [Mongoose: an Embeddable Web Server in C](http://www.linuxjournal.com/article/10680)