Removed checking for a fallback control file

- This has never worked and can't work in the future, products
  are too different and using that file might cause more issues
  than good (gh#yast/yast-installation#86)
Added changes file entry and new version
More effective searching for available file

- Credits: jreidinger
Handling incorrectly set custom control file
Merge pull request #134 from kobliha/no_fallback_control_file

Removed checking for a fallback control file
Add CONTRIBUTING.md

See:

  * https://github.com/blog/1184-contributing-guidelines
  * http://lists.opensuse.org/yast-devel/2013-10/msg00052.html