docs: declare the dependency on the collection community.crypto

The role uses openssl_privatekey, openssl_csr, x509_certificate and x509_certificate_info, but neither declared
that collection anywhere nor mentioned it in the readme. A standalone role cannot express collection dependencies in
its metadata, so a requirements file next to the readme section is the usual way.

The section also documents two requirements that arose from the recent changes and were undocumented as well. Facts
have to be gathered, because the package names, the trust store anchor and the update command are resolved by
distribution, os_family and architecture. And the role needs become, since it writes into /etc and updates the trust
store.

No version is pinned. All used modules exist since community.crypto 1.0.0, so a lower bound would be arbitrary.

Co-authored-by: Copilot <copilot@github.com>

README.md

@@ -4,6 +4,19 @@ This Ansible role can be used to create a root and intermediate certificate auth
 them. Additionally offers the ansible role the feature to import the certificates of the authority into the systems
 trust store.
 
+## Requirements
+
+The role relies on the modules of the collection `community.crypto`.
+
+```bash
+ansible-galaxy collection install -r requirements.yaml
+```
+
+Facts must be gathered, because the names of the required python packages, the location of the trust store anchor and
+the command to update the trust store are looked up by `ansible_facts['distribution']`, `ansible_facts['os_family']`
+and `ansible_facts['architecture']`. Archlinux, Debian and RedHat based distributions are supported. Furthermore the
+role writes into `/etc` and updates the systems trust store, so it has to be executed with `become: true`.
+
 ## Examples
 
 The following minimal example creates a root and intermediate certificate authority and issues a client certificate from