Added a README for the unpacker

Signed-off-by: Valentin Popov <valentin@popov.link>

unpacker/README.md

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+# NRes Game Resource Unpacker
+
+At the moment, this is a demonstration of the NRes game resource unpacking algorithm in action.
+It unpacks 100% of the NRes game resources for the game "Parkan: Iron Strategy".
+The unpacked resources can be packed again using the [packer](../packer) utility and replace the original game files.
+
+__Attention!__
+This is a test version of the utility.
+It overwrites existing files without asking.
+
+## Building
+
+To build the tools, you need to run the following command in the root directory:
+
+```bash
+cargo build --release
+```
+
+## Running
+
+You can run the utility with the following command:
+
+```bash
+./target/release/unpacker /path/to/file.ex /path/to/output
+```
+
+- `/path/to/file.ex`: This is the file containing the game resources that will be unpacked.
+- `/path/to/output`: This is the directory where the unpacked files will be placed.
+
+## How it Works
+
+The structure describing the packed game resources is not fully understood yet.
+Therefore, the utility saves unpacked files in the format `file_name.file_index` because some files have the same name.
+
+Additionally, an `index.json` file is created, which is important for re-packing the files.
+This file lists all the fields that game resources have in their packed form.
+It is essential to preserve the file index for the game to function correctly, as the game engine looks for the necessary files by index.
+
+Files can be replaced and packed back using the [packer](../packer).
+The newly obtained game resource files are correctly processed by the game engine.
+For example, sounds and 3D models of warbots' weapons were successfully replaced.