Update readme.

Signed-off-by: Andrei Sandu <[email protected]>

README.md

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 # versionize
+
+## Important note
+
+This crate is currently used for cross-version serialization with the [Firecracker snapshot-restore dev preview](https://github.com/firecracker-microvm/firecracker/tree/v0.23.0), but has not been tested for other use cases. It should be considered **experimental software** outside the Firecracker context. It’s likely that this crate will see both interface and implementation changes in the future.
+
+## Description
+
 Provides version tolerant serialization and deserialization facilities and implements a persistent storage format for state snapshots.