user-manual-hello.xml:  Filling out the Hello World example.

Building on the blog post written by Eren TUrkay, this chapter
explains the Hello World example as generically as possible.

Signed-off-by: Bill Traynor <wmat@alphatroop.com>
user-manual-intro.xml: Expanded intro significantly.

Added serveral bits around the source code layout,
obtaining the source code and the concepts the BitBake
was based on.

Signed-off-by: Bill Traynor <wmat@alphatroop.com>