Update Prerequisites a bit

docs/prerequisites.md

@@ -4,7 +4,9 @@
 
 - [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`).
 
-- the [Ansible](http://ansible.com/) program being installed on your own computer. It's used to run this playbook and configures your server for you. Take a look at [our guide about Ansible](ansible.md) for version requirements or alternative ways to run Ansible.
+- the [Ansible](http://ansible.com/) program being installed on your own computer. It's used to run this playbook and configures your server for you. Take a look at [our guide about Ansible](ansible.md) for [version requirements](ansible.md#supported-ansible-versions) or alternative ways to run Ansible.
+
+- either the `dig` tool or `python-dns` installed on your own computer. Used later on, by the playbook's [services check](maintenance-checking-services.md) feature.
 
 - an HTTPS-capable web server at the base domain name (`<your-domain>`) which is capable of serving static files (unless you decide to use DNS SRV records for [Server Delegation](howto-server-delegation.md))