Readme update

I started this branch several days ago but since then you changed the README file. It's a proposal but I changed it to markdown, it seems to be the README stadard now. In addition the right HTML headings are used (but I may be punctilious here ^^).

I also added a code climate badge. It's still referencing the old repository, I can update it but it doesn't seem to work for now.

I also added a link to the online RDoc and instrcutions for contributions. Otherwise, I kept what you changed in the new README.