Switching README.md with README.txt

I saw that README.md was just a symbolic link to README.txt
Unfortunately, when a Github user click on README.md, it just displays ```README.txt``` instead of displaying directly README.txt.
However, if we display the README.txt through Github, **we did not enjoy the Markdown syntax** because the extention of the file was .txt.
That's why I chose to swich the both files to get a cool README interface.