add README

data/storage/README.md

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+# DuckDB files
+DuckDB v0.10 enables the creation of storage database files, ensuring seamless compatibility for subsequent versions to read files generated by older versions. *Note: it is recommended to compress large database files.*
+
+For further details, please refer to: [Storage & Compatibility](https://duckdb.org/docs/internals/storage#compatibility)
+
+## How to gzip a database file
+```
+gzip <filename.db>
+```
+
+## How to unzip and load a database file in the test runner
+```
+unzip <path/to/filename.db.gz> <extraction/path/to/filename.db>
+
+load <extraction/path/to/filename.db> readonly
+```
+
+or by using the default extraction path `__TEST_DIR__/` (temporary space)
+```
+unzip <path/to/filename.db.gz>
+
+load __TEST_DIR__/<filename.db> readonly
+```