Fixing broken link in Readme

There is a broken link in the `README.md`...

On Line 83 of `README.md`... there is a link to goto the `Contributing.md` document.  However, the file is named as `Contributing.md`, but the link was looking for a file named `CONTRIBUTING.md`.  So I renamed the file to be all caps... `CONTRIBUTING.md`.

Please review, and let me know if need any further changes.
Thanks very much!