Add list of online services + maintainers to README.md

At the tango kernel meeting on 23 Sep 2021, it was requested to have a list of the online services we use, as well as the maintainers. This will help to point people in the right direction, and also help us find services with too few maintainers.

https://gitlab.com/tango-controls/tango-kernel-followup/-/tree/main/2021/2021-09-23