Specify that cron is likely required on the server

When using Let's Encrypt SSL certificates, a cronjob is set up to
automatically renew them. Though it does require a `cron`-compatible
program on the server.

This fixes the error that is caused by the `/etc/cron.d` directory
not existing and the `ansible-cron` module trying to write out a
file there -- without checking if the directory exists first.

docs/prerequisites.md

@@ -4,6 +4,8 @@
 
 - [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. *This can be ignored if you use your own SSL certificates.*
+
 - 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 more information, as well as [version requirements](ansible.md#supported-ansible-versions) and 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.