Write README in markdown

[xgaia]

Move README to README.md for GitHub viewing and write it in markdown.

I also replace apt-get and yum command into to apt and dnf

[dhimmel]

Would be nice to have GitHub render the README as markdown by adding a .md extension. It looks like much of the readme would actually render sufficiently in markdown, even without these changes.

@xgaia any reason why this project doesn't use an .md?