Add first timers only badge to readme

[wooorm]

Subject of the issue

We have a label for it, and this issue is for first timers, but we should announce that in the readme, next to travis and hacktoberfest.

Steps to reproduce

View readme on GitHub.

Expected behaviour

There should be a nice badge. Something like:

[![First timers friendly][first-timers-badge]][first-timers]

<!-- and somewhere later with the other definitions -->

[first-timers-badge]: https://img.shields.io/badge/first--timers--only-friendly-blue.svg

[first-timers]: https://www.firsttimersonly.com/

Actual behaviour

There is no badge 😢