Update documentation for built-in renderers

docker/README.md

@@ -0,0 +1,17 @@
+## Build
+To use `cross` to compile for Raspberry Pi you first need to build the docker image.
+Use these commands to build the images needed.
+
+**NOTE:** Run these commands inside the `docker` folder. This is needed since `docker`
+uses surrounding directories as "context" for building the image, which means it'll
+copy the entire `target` directory.
+
+### Raspberry Pi 2/3/4 (32 bits)
+```
+$ docker build -t iced-rs/armv7 -f Dockerfile.armv7-unknown-linux-gnueabihf .
+```
+
+### Raspberry Pi 3/4 (64 bits)
+```
+$ docker build -t iced-rs/aarch64 -f Dockerfile.aarch64-unknown-linux-gnu .
+```