Add build instructions

BUILD.md

@@ -0,0 +1,72 @@
+# Building
+
+The build for this is unfortunately really complicated. Use this file to
+describe the build process.
+
+## Overview
+
+Starting from a bare repository, you need to do the following things to get
+a translated MOTHER 1+2 ROM:
+
+1. Get an original MOTHER 1+2 ROM and EarthBound ROM (not provided here).
+    1. Name the MOTHER 1+2 ROM `m12fresh.gba` and place it in the repository
+       folder.
+    2. Name the EarthBound ROM `eb.smc` and put it in the same folder.
+2. Build the build (see below).
+3. Build the script (see below).
+4. Build the hack and insert it into the ROM (see below).
+
+## Building the build
+
+This only has to be done once, assuming you're not making changes to the
+build tools.
+
+You will need Visual Studio to build the build. The Community edition is free:
+
+https://www.visualstudio.com/vs/community/
+
+Download and install that. Then, for each of the following solution files,
+
+1. ScriptTool\ScriptTool.sln
+2. SymbolTableBuilder\SymbolTableBuilder.sln
+3. compiled\Amalgamator\Amalgamator.sln
+
+do the following:
+
+1. Open the .sln file with Visual Studio
+2. In the Solution Explorer pane, right-click the solution and "Restore
+   NuGet Packages"
+3. Once that finishes, right-click the solution again and "Build Solution"
+
+## Building the script
+
+"The script" refers to the translation files in the `working` folder.
+
+You will use ScriptTool.exe (one of the things you built earlier) to build the
+script. It will be located in ScriptTool\ScriptTool\bin\Debug\ScriptTool.exe.
+
+This is a command-line tool. Syntax:
+
+`ScriptTool.exe -compiled -main -misc <working folder path>`
+
+There's no batch file for this, although I ought to make one. You will have to
+run this every time you make a change to the script.
+
+## Building the hack
+
+After the script is built, you need to build the hack code and insert it into
+the MOTHER 1+2 ROM file. This is taken care of by `insert.bat`; it will generate a file called `m12.gba` with everything ready.
+
+There's another one-time pre-requisite to take care of before you can run it.
+
+### C code
+
+Some of the hack code is written in C and cross-compiled to ARM with GCC.
+In addition to armips described above, you will need to install this
+GCC toolchain:
+
+[gcc-arm-none-eabi-7-2017-q4-major-win32](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads)
+
+It was at version 7-2017-q4-major at the time of writing, but I imagine newer
+versions work just fine. Download and run the installer linked above. Make sure
+you add the path to your environment variable.