Add some style

While we are still here, I'd like to mention two issues:

1. There are no branches in this repo yet, so the paragraph on branches might mislead someone. Maybe it is better to remove it for now and add back when there are actual branches? Contribution guidelines will probably have to be amended anyway when work on the second edition starts.
2. Typing two spaces between sentences is [weird](http://practicaltypography.com/one-space-between-sentences.html). Well, thanks to Markdown, only ones _editing_ the file see that, but nevertheless…