Missing contributor credits

Problem/Motivation

When I merge commits, I often forget to add the appropriate credits to the appropriate people. My public apologies. It's not on purpose as I am willing to give credits to everybody who contributed. It's just that I neglect/forget to check that the commits contain the appropriate things...

Proposed resolution

I'll add currently missing credits in a commit for this "issue". It will also add a "credits" sub-section to the README.md to credit both people and additional supporting organizations. Please post a message here if you think I forgot you (sorry in advance) or if you have better suggestions for some credits.

So far, I (for)got:

- Abhishek Gupta [abhishek_gupta1](https://www.drupal.org/u/abhishek_gupta1)
  - Issue #3455366
- Antonio De Marco [ademarco](http://nuvole.org/)
  - Issue #3479118
- [arousseau](https://www.drupal.org/u/arousseau) ([insite.coop](https://www.insite.coop))
  - Issue #3486164
- Joachim Noreiko [joachim](https://www.drupal.org/u/joachim)
  - Issue #3374867
- [mortona2k](https://www.drupal.org/u/mortona2k)
  - Issue #3449832
  - Issue #3049317