[doc] Reflow and reorganize CONTRIBUTING.md guidelines
[doc] Reflow and reorganize CONTRIBUTING.md guidelines

The contributing guidelines were a bit too cumbersome to read.

Tried to reflow it and push larger blobs of text towards the end of the
document, add a little lighter tone with emojis, emphasize we appreciate
contributions right from the start.

Also took inspiration from the Atom and Ruby on Rails contributing
guidelines mentioned in Github's help pages.

reactor/reactor-core