doc(rootfs): add rootfs README

rootfs/README.md

+# RootFS
+
+This directory stores all files that should be copied to the rootfs of a
+Docker container. The files should be stored according to the correct
+directory structure of the destination container. For example:
+
+```
+rootfs/bin -> /bin
+rootfs/usr/local/share -> /usr/local/share
+```
+
+## Dockerfile
+
+A Dockerfile in the rootfs is used to build the image. Where possible,
+compilation should not be done in this Dockerfile, since we are
+interested in deploying the smallest possible images.
+
+Example:
+
+```Dockerfile
+FROM alpine:3.2
+
+COPY . /
+
+ENTRYPOINT ["/usr/local/bin/boot"]
+```