Add a note about beta/pre-release distros

docs/prerequisites.md

@@ -1,6 +1,6 @@
 # Prerequisites
 
-- **CentOS** (7.0+), **Debian** (9/Stretch+) or **Ubuntu** (16.04+) server. This playbook can take over your whole server or co-exist with other services that you have there.
+- **CentOS** (7.0+), **Debian** (9/Stretch+) or **Ubuntu** (16.04+) server. 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.
 
 - [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`).