FAQ four years old

[guettli]

The FAQ is 4 years old:

https://github.com/argoproj/gitops-engine/blob/master/docs/faq.md

I am unsure if the answers are still valid. It would be great if you could update the FAQ (that is not something a new contributor can do).

[jmeridth]

#647 updates the Slack link to argo-cd-contributors channel in CNCF Slack from gitops channel in Kubernetes Slack.