Add README

Author: qzed

README.md

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+# GRUB AArch64 Image for Surface Pro X
+
+Docker container to build a (self-contained) GRUB AArch64 image for the Surface Pro X on a host with different architecture (e.g. x86).
+
+
+## Basic Instructions
+
+To build the container (named `aarch64-grub`) and a known-good image simply run `./mkimage.sh` from this directory.
+
+## Manual Instructions
+
+To build the container, run
+```
+docker build . -t aarch64-grub
+```
+where `aarch64-grub` is an arbitrary container name.
+
+To run arbitrary grub commands run
+```sh
+docker run [docker options...] aarch64-grub <command> [command options...]
+```
+Note that you have to prefix the standard `grub` commands with `aarch64-`.
+For example, to generate a fully self-contained image in the current directory, run
+```sh
+docker run --rm                     \
+    -v "${PWD}/":/output            \
+    aarch64-grub                    \
+    aarch64-grub-mkimage            \
+        -O arm64-efi                \
+        -o /output/bootaa64.efi     \
+        --prefix=                   \
+        $(cat modules.txt)
+```