Mention that root access is a requirement

Fixes #396 (Github Issue).

docs/prerequisites.md

@@ -2,6 +2,8 @@
 
 - An x86 server running **CentOS** (7 only for now; [8 is not yet supported](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/300)), **Debian** (9/Stretch+) or **Ubuntu** (16.04+). This playbook doesn't support running on ARM ([see](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/299)), however a minimal subset of the tools can be built on the host, which may result in a working configuration, even on a Raspberry pi (see [Self-Building](self-building.md)). We only strive to support released stable versions of distributions, not betas or pre-releases. This playbook can take over your whole server or co-exist with other services that you have there.
 
+- `root` access to your server (or a user capable of elevating to `root` via `sudo`).
+
 - [Python](https://www.python.org/) being installed on the server. Most distributions install Python by default, but some don't (e.g. Ubuntu 18.04) and require manual installation (something like `apt-get install python`).
 
 - a `cron`-like tool installed on the server such as `cron` or `anacron` to automatically schedule the Let's Encrypt SSL certificates's renewal. *This can be ignored if you use your own SSL certificates.*