Update Stage 0 Firmware readme
# stage0
-This is -- or, rather, will be -- a minimal firmware for use with QEMU that just
-enters long mode with no extra features.
+This is a minimal firmware for use with QEMU and compatible VMMs that puts the
+CPU into the desired state, enters long mode, retrieves required configuration
+information (E820 memory layout and ACPI tables) and jumps into the kernel entry
+point.
+
+The E820 memory map and ACPI tables are read using the
+[QEMU Firmware Configuration (fw_cfg)](https://www.qemu.org/docs/master/specs/fw_cfg.html)
+device.
To create the BIOS blob, you need to turn it into a headerless file with
-`objcopy`:
+`objcopy` after the binary was built:
+
+```shell
+cargo build --release
+objcopy --output-format binary target/x86_64-unknown-none/release/oak_stage0 \
+ target/x86_64-unknown-none/release/stage0.bin
+```
+
+This can also be done in one step using cargo-binutils:
```shell
-objcopy --output-format binary target/x86_64-unknown-none/{debug,release}/oak_stage0 stage0.bin
+cargo objcopy --release -- --output-format binary \
+ target/x86_64-unknown-none/release/stage0.bin
```
The resulting `stage0.bin` should be exactly `BIOS_SIZE` (defined in
