add README.md

README.md

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+## Building MCU Code
+Build the Docker Image
+```
+docker build \
+  --file klipper.Dockerfile \
+  --target mcu \
+  --tag klipper-mcu \
+  .
+```
+
+Build the mcu binary.
+
+This example mounts an existing build config at `klipper/.config`. One may want to initially create this file via `make menuconfig`
+```
+docker run \
+  --rm \
+  --volume /opt/docker-sparkcube/config/config.udoo:/opt/klipper/.config \
+  --volume /opt/docker-sparkcube/out:/opt/klipper/out \
+  --interactive \
+  --tty \
+  klipper-mcu \
+    bash -c "cd /opt/klipper; make"
+```