Add a warning about ARM, de-brand

Closes coop-cloud/organising#25

docs/index.md

@@ -15,11 +15,21 @@ HOWTOs:
  - [Deploy an HTML site](howto/html-website.md)
  - [CoCl-ise an app](howto/convert-app.md)
 
+!!! warning "A note about ARM"
+    Not all applications currently support all ARM computers (like PINE64 and
+    Raspberry Pi): e.g. [Traefik will work on ARMv6 & ARM64][traefik], [the
+    official MariaDB app doesn't work on any ARM boards][mariadb]. It might be possible
+    to use third-party ARM images with separate `compose.yml` files, but we
+    haven't exlplored this rabbit-hole much yet.
+
 ## Definitions
 
 CoCl is a **philosophy**.
 
- - .. to get started, you create a **server**, e.g. a new Hetzner VPS, including
+ - .. to get started, you create a **server**, e.g. a new VPS, including
    your local configuration to connect to it.
  - .. then you install an **app** like wordpress or nextcloud, which is made up of multiple **services**
  - .. and configure your app.
+
+[traefik]: https://hub.docker.com/_/traefik?tab=tags
+[mariadb]: https://hub.docker.com/_/mariadb?tab=tags