text: Title: add first-timers-welcome badge to readme
  Body: Very easy, we need a badge to show that we welcome contributions from first-timers.
  
  The code for the badge can be taken from here:
  https://www.firsttimersonly.com/
  
  and it is actually this one:
  [![first-timers-only](https://img.shields.io/badge/first--timers--only-friendly-blue.svg?style=flat-square)](https://www.firsttimersonly.com/)
  