Add readme

readme.md

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+# docker-qemu-arm
+
+This Debian-based image allows you to easily build cross-platform images.
+It's been tested with GitLab CI on gitlab.com, but it should work anywhere that docker-in-docker already works, and with a binfmt_misc enabled kernel.
+
+## Example Usage
+
+This GitLab example should give you an idea of how to use the image.
+
+Dockerfile
+```
+FROM alpine
+
+RUN echo "Hello, my CPU architecture is $(uname -m)"
+```
+
+.gitlab-ci.yml
+```
+variables:
+  DOCKER_HOST: tcp://docker:2375/
+
+build:
+  image: jonoh/docker-qemu-arm
+  stage: build
+  services:
+    - docker:dind
+  before_script:
+    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  script:
+    - docker buildx build --platform linux/arm/v7,local --pull -t "$CI_REGISTRY_IMAGE" --push .
+```
+
+And the (partial) output:
+```
+#6 [linux/amd64 2/2] RUN echo "Hello, my CPU architecture is $(uname -m)"
+#6 0.120 Hello, my CPU architecture is x86_64
+#6 DONE 0.3s
+
+#8 [linux/arm/v7 2/2] RUN echo "Hello, my CPU architecture is $(uname -m)"
+#8 0.233 Hello, my CPU architecture is armv7l
+#8 DONE 0.2s
+```