Indicate that Ubuntu Bionic (18.04) is not supported yet

We have 2 blockers that prevent us from adding support:

- the Docker CE repository does not publish a `docker-ce` package
in the `stable` channel. It's still in `edge`
(can be worked around by using `edge`, but we'd better not)

- Docker bind propagation has troubles on Docker CE 18.05,
which breaks matrix-synapse.service from starting, as it wants to do
a `:slave` mount. See https://github.com/moby/moby/issues/37032

README.md

@@ -58,7 +58,7 @@ Special thanks goes to:
 
 ## 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** (anything 16.04+, except 18.04 for now) server. This playbook can take over your whole server or co-exist with other services that you have there.
 
 - the [Ansible](http://ansible.com/) program, which is used to run this playbook and configures everything for you