README: add build instructions

README.md

@@ -2,9 +2,36 @@
 
 Game Boy (DMG) emulator [Peanut-GB](https://github.com/deltabeard/Peanut-GB) on the Raspberry Pi RP2040 microcontroller, using an ILI9225 screen.
 
-Runs at more than 70 fps without audio emulation. WIth frame skip and interlacing, can run at up to 120 fps.
+Runs at more than 70 fps without audio emulation. With frame skip and interlacing, can run at up to 120 fps.
 
-![1](https://user-images.githubusercontent.com/3747104/162945331-605747fb-e48e-4b29-8007-9947afa29597.jpg)
+![Photo of a Raspberry Pi Pico playing Pokemon Red](https://user-images.githubusercontent.com/3747104/162945331-605747fb-e48e-4b29-8007-9947afa29597.jpg)
+
+# Building
+
+The [Raspberry Pi Pico SDK](https://github.com/raspberrypi/pico-sdk) is required to build this project. Make sure you are able to compile an [example project](https://github.com/raspberrypi/pico-examples#first--examples) before continuing.
+
+The file `./src/rom.c` is required for this project to compile. This contains the ROM data that will be played by Peanut-GB on the RP2040. To generate this file, the program `xxd` is required. `xxd` is a tool that can convert binary files to a valid C header file, and is packaged with [Vim](https://www.vim.org/) and [NeoVim](https://neovim.io/), prepackaged within the [w64devkit](https://github.com/skeeto/w64devkit) development environment, or can be compiled from source [from the vim repository](https://github.com/vim/vim/tree/master/src/xxd).
+
+Steps:
+1. Convert your `*.gb` or `*.gbc`† file to a C header file by executing the following command in a terminal or command prompt:
+```sh
+xxd -i rom.gb rom.c
+```
+
+† *Note that Game Boy Color (GBC) games are not supported with Peanut-GB or RP2040-GB. On load, games that require a GBC hardware will typically display an error message as they will be forced to boot into DMG mode.*
+
+2. Open the `rom.c` file in a text editor, and replace the first line with:
+```
+#include <pico/platform.h>
+const unsigned char __in_flash("rom") rom[] = {
+```
+Since RP2040-GB is configured to run from internal RAM for performance, the `__in_flash` attribute stops the ROM data from also being copied to the RAM because most ROMs are larger than the available RAM on the RP2040. We also additionally define `const` as this is read-only data.
+
+3. Copy this `rom.c` file to the `src` folder.
+
+The project should now compile with your ROM builtin.
+
+Depending on the license of the ROM that you have built into this project, the output RP2040 binary may not be redistributable under the terms of you local copyright law.
 
 ## Future work