Split README into a bunch of files in docs/

docs/configuring-dns.md

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+# Configuring your DNS server
+
+To set up Matrix on your domain, you'd need to do some DNS configuration.
+
+To use an identifier like `@<username>:<your-domain>`, you don't actually need
+to install anything on the actual `<your-domain>` server.
+
+All services created by this playbook are meant to be installed on their own server (such as `matrix.<your-domain>`).
+
+To accomplish such a "redirect", you need to instruct the Matrix network of this by setting up a DNS SRV record.
+The SRV record should look like this:
+- Name: `_matrix._tcp` (use this text as-is)
+- Content: `10 0 8448 matrix.<your-domain>` (replace `<your-domain>` with your own)
+
+Once you've set up this DNS SRV record, you should create 2 other domain names (`matrix.<your-domain>` and `riot.<your-domain>`) and point both of them to your new server's IP address (DNS `A` record or `CNAME` is fine).
+
+This playbook can then install all the services on that new server and you'll be able to join the Matrix network as `@<username>:<your-domain>`, even though everything is installed elsewhere (not on `<your-domain>`).
+
+When ready to proceed, continue with [Configuring this Ansible playbook](configuring-playbook.md).