Add LICENSE.md file and improved a little the README

[valerio-bozzolan]

Few changes here:

• added a LICENSE.md file with your license (GNU GPL v3+) in order to allow GitHub to recognize the project's license
• improved a little the README (and converted to markdown) with a link to the official website, documentation and to cite the author

P.S.

Thank you so much for this software. I've seen it in action for elections in https://www.wikimedia.it/. I'd like to make some contributions to simplify the user interface and introduce internationalization with some standards like GNU Gettext or I don't know but, for now, just this pull request. Cheers!