Add notes about running Ansible on Ubuntu 20.04

Discussed in #669 (Github Issue).

docs/ansible.md

@@ -11,6 +11,8 @@ If your local computer cannot run Ansible, you can also run Ansible on some serv
 
 Ansible 2.7.0 or newer is required.
 
+Ubuntu (at least 20.04) ships with a buggy version (see this [bug](https://bugs.launchpad.net/ubuntu/+source/ansible/+bug/1880359)), which can't be used in combination with a host running new systemd (more detaisl in [#517](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/517), [#669]([669](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/669))). If this problem affects you, you can: avoid running Ubuntu 20.04 on your host; run Ansible from another machine targeting your host; or try to upgrade to a newer Ansible version (see below).
+
 
 ## Checking your Ansible version
 

docs/prerequisites.md

@@ -1,6 +1,12 @@
 # Prerequisites
 
-- 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+), **Ubuntu** (16.04+), or **Archlinux**. 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 [Alternative Architectures](alternative-architectures.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.
+- An **x86** server running one of these operating systems:
+  - **CentOS** (7 only for now; [8 is not yet supported](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/300))
+  - **Debian** (9/Stretch+)
+  - **Ubuntu** (16.04+, although [20.04 may be problematic](ansible.md#supported-ansible-versions))
+  - **Archlinux**
+
+This playbook doesn't support running on ARM (see [this issue](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 [Alternative Architectures](alternative-architectures.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`).