Add a note to the readme for Apple Silicon users

In an ideal world this would be resolved by having the base image
built for ARM as well as amd64, but without ARM runners for GitHub
Actions this is infeasibly slow (I aborted a test build after 30
minutes - it takes less than a minute to do a build on my M1 Pro
laptop).

README.md

@@ -19,6 +19,17 @@ to monitor the containers.
 
 To start all containers (and rebuild any which may have changed):
 
+> [!TIP]
+> **Apple Silicon Users**
+> You'll probably want to build your own version of the base images as well, as
+> the ones we build are amd64 only, and so painfully slow on ARM devices. This
+> is unlikely to be resolved until GitHub Actions has ARM runners.
+>
+> ```sh
+> docker build -t ghcr.io/emfcamp/website-base -f ./docker/Dockerfile.base .
+> docker build -t ghcr.io/emfcamp/website-base-dev -f ./docker/Dockerfile.base-dev .
+> ```
+
 ```
 docker compose build --parallel
 docker compose up