Add a basic Readme with build instructions

Author: phil-opp

real_mode/first_stage/README.md

@@ -0,0 +1,16 @@
+# First Stage: Bootsector
+
+This executable needs to fit into the 512-byte boot sector, so we need to use all kinds of tricks to keep the size down.
+
+## Build Commands
+
+1. `cargo build --release -Zbuild-std=core --target x86-16bit.json -Zbuild-std-features=compiler-builtins-mem`
+2. `objcopy -I elf32-i386 -O binary target/x86-16bit/release/first_stage target/disk_image.bin
+
+To run in QEMU:
+
+- `qemu-system-x86_64 -drive format=raw,file=target/disk_image.bin`
+
+To print the contents of the ELF file, e.g. for trying to bring the size down:
+
+- `objdump -xsdS -M i8086,intel target/x86-16bit/release/first_stage`