Replaced ReadMe.txt with README.md and Added CONTRIBUTING.md

Replaced ReadMe.txt with [README.md](https://github.com/rlove/DUnitX/blob/feature/MarkdownFiles/README.md)
Added [CONTRIBUTING.md](https://github.com/rlove/DUnitX/blob/feature/MarkdownFiles/CONTRIBUTING.md)

[md is Markdown format](https://help.github.com/articles/github-flavored-markdown)

[How CONTRIBUTING.md works on GitHub.](https://github.com/blog/1184-contributing-guidelines)


https://github.com/rlove/DUnitX/tree/feature/MarkdownFiles