add INSTALL file

INSTALL.md

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+First, you must put a Pokémon Emerald (US) ROM in the root directory of the repository and name it `base_emerald.gba`. It should have a SHA1 checksum of `f3ae088181bf583e55daf962a92bb46f4f1d07b7`. Then, follow the OS-specific instructions below.
+
+# Linux
+
+TBD
+
+# Windows
+
+Install [**devkitARM**](http://devkitpro.org/wiki/Getting_Started/devkitARM).
+
+Then get the compiled tools from https://github.com/YamaArashi/pokeruby-tools. Copy the "tools" folder over the "tools" folder in your pokeemerald directory.
+
+You can then build pokeemerald using "make" in the MSYS environment provided with devkitARM.