Change styleguide by pull request instead

See the rendered Markdown: https://github.com/henrik/devbook/blob/styleguide-by-pullreq/styleguide/README.md#how-to-change-this-styleguide

If all six of us give a :thumbsup: or :punch:, whoever supports or forfeits last can go ahead and merge this.

I say :thumbsup: ;)

- [x] HN
- [x] JK
- [x] AR
- [x] KP
- [x] TS
- [x] VA