Document how to run benchmarks

From downloading the tar archive, extracting, and running the benchmark via `cargo bench`.

benches/README.md

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+# Benchmarks
+
+Measure the throughput of reading/writing GeoTIFF files using this library!
+
+Steps to run:
+
+1. Download the tar archive of GeoTIFF files, placing them in the `resources/` folder
+
+   ```bash
+   wget https://s3.us-east-2.amazonaws.com/geotiff-benchmark-sample-files/geotiff_sample_files.tar.gz -P resources
+   ```
+
+2. Extract the tar archive
+
+   ```bash
+   tar --extract --verbose --file resources/geotiff_sample_files.tar.gz
+   ```
+
+3. Run the benchmarks
+
+    ```bash
+    cargo bench
+    ```
+
+## Results
+
+TODO
+
+## References
+- https://github.com/kokoalberti/geotiff-benchmark