Add CI README, explaining how to update docker images (#162)

ci/README.md

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+# Keeping CI images up to date
+
+## Re-building an image after updating the docker file
+
+Run `docker build -t snmallocciteam/$IMG:latest -f ci/$IMG .` from the root of the repo,
+where `$IMG` is the image you want to rebuild, for example `build_linux_x64`
+
+If you are building a multiarch image, ie. an image targeting an architecture other than
+the one you are running, you will need to install the qemu handler before you can build:
+`sudo docker run --rm --privileged multiarch/qemu-user-static:register --reset`
+
+## Pushing the updated docker image to Docker Hub
+
+Run `docker push snmallocciteam/$IMG:latest`
+
+## Permissions
+
+You must be part of the [snmalloc ci team](https://hub.docker.com/orgs/snmallocciteam) to
+push updated images. Contact @mjp41, or @achamayou to be given access.