Add Readme.md security warning

[misolietavec]

I discovered the minergate-cli (cryptocurrency miner) running in one of tmpnb containers, fully eating all the 16 cores on server. There should be a strong warning in "Quick start" session of Readme.md that it is possible to install and run arbitrary softvare as root in the container with that simple configuration. It is very dangerous, especially if running containers several days...

[misolietavec]

Hmm, the warning itself is not helpful. Notebooks in containers will start as root (can I start the docker run ... as ordinary user?). This can be cured by adding the option --container-user=jovyan to orchestrate.py in the sample command in section "Quick start". May I suggest to add this to Readme.md? I think, this can help many users.

[rgbkrk]

Sure, feel free to make a PR.

[misolietavec]

PR made, merged. I can close the issue.

[willingc]

Thanks @misolietavec!