Readme rework

[roipoussiere]

As suggested in a comment on the PR #53, make changes on readme in the dev branch.

This includes:

• move fork explanation to the end of readme (better readability);
• add license section;
• move badges to the top (quick access);
• improve titles structure;
• add summary;
• fix typos and other minor improvements;
• add contribution guide with some love inside.

To a better understanding, all of these steps are done in separated commits but fill free to squash them.

[Ionaru]

Couple of small things I've noticed, other than that it looks good!

[roipoussiere]

Thanks for the suggestion! I didn't know that we can do this kind of list with GFM, by local markdown editor don't render them like that.