Mention specs on the Prerequisites page

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/682

docs/prerequisites.md

@@ -2,7 +2,7 @@
 
 To install Matrix services using this Ansible playbook, you need:
 
-- (Recommended) An **x86** server running one of these operating systems:
+- (Recommended) An **x86** server ([What kind of server specs do I need?](faq.md#what-kind-of-server-specs-do-i-need)) 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 or newer)
   - **Ubuntu** (16.04 or newer, although [20.04 may be problematic](ansible.md#supported-ansible-versions))