docs: Updating my gitignore.

Added a few more files to gitignore file to allow document
generation testing.

Signed-off-by: Bill Traynor <wmat@alphatroop.com>
README: Removed YP specific content.

Reusing the README file from the Yocto Project Documentation
directory.  Removed the YP specific content and changed to be
specific to BitBake.

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