Add a section to the readme on how to recompile

[sudo-sandwich]

I am unable to recompile identically to the original rom using devkitARM.

arm-none-eabi-gcc -o rom.elf -specs=gba.specs rom.s

Gives the following output:

rom.s: Assembler messages:
rom.s:42584: use of r13 is deprecated
rom.s:42608: use of r13 is deprecated
rom.s:42623: use of r13 is deprecated
c:/devkitpro/devkitarm/bin/../lib/gcc/arm-none-eabi/11.1.0/../../../../arm-none-eabi/bin/ld.exe: c:/devkitpro/devkitarm/bin/../lib/gcc/arm-none-eabi/11.1.0/../../../../arm-none-eabi/lib/gba_crt0.o: in function `CIDExit':
(.crt0+0x20c): undefined reference to `main'
collect2.exe: error: ld returned 1 exit status