Adding "Adopter" section to Github Repository (pr: #11503)

[Gabriel Goller]

Adding the "Adopters" section to the readme.md file in the github repository will increase our score on the CLOMonitor(issue: #11462).

We already have a "Some of our users include" section on the website prometheus.io (on the bottom).

I could find some other adopters online (there is a link to a blog post from the company explaining how they use it):
* [SoundCloud](https://developers.soundcloud.com/blog/prometheus-monitoring-at-soundcloud)
* [Uber](https://www.uber.com/blog/virginia/tag/m3/)
* [Robinhood](https://robinhood.engineering/upgrading-scaling-airflow-at-robinhood-5b625dfaa2ee)
* [Slack](https://slack.engineering/recommend-api/)

The thing is I don't know about the legal aspects of this... can we simply include them in the "Adopters" section or do we need to ask for permission?

Gabriel