Mention that ARM is not supported

Feel free to reword but we have had a couple of issues created now asking about Raspberry Pi support.

docs/prerequisites.md

@@ -1,6 +1,6 @@
 # Prerequisites
 
-- **CentOS** (7.0+), **Debian** (9/Stretch+) or **Ubuntu** (16.04+) server. We only strive to support released stable versions of distributions, not betas or pre-releases. This playbook can take over your whole server or co-exist with other services that you have there.
+- An x86 server running **CentOS** (7.0+), **Debian** (9/Stretch+) or **Ubuntu** (16.04+). This playbook doesn't support running on ARM so it won't work on a Raspberry Pi. We only strive to support released stable versions of distributions, not betas or pre-releases. This playbook can take over your whole server or co-exist with other services that you have there.
 
 	- for ancient distributions like **CentOS 7.0**, we recommend that you do a manual systemd-journald adjustment. Because the Synapse chat server is incredibly chatty when it comes to logging (here's [one such issue](https://github.com/matrix-org/synapse/issues/4751) describing the problem), be advised that systemd's journald default logging restrictions may not be high enough to capture all log messages generated by Synapse. This is especially true if you've got a busy (Synapse) server. We advise that you manually add `RateLimitInterval=0` and `RateLimitBurst=0` under `[Storage]` in the `/etc/systemd/journald.conf` file, followed by restarting the logging service (`systemctl restart systemd-journald`).