readme: add initial readme

Signed-off-by: Peng Tao <[email protected]>

README.md

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+# dbs-snapshot
+
+`dbs-snapshot` is a fork of [Firecracker microvmm's](https://github.com/firecracker-microvm/firecracker) `snapshot` crate. It was done so because the Firecracker maintainers [have no plan to publish the crate on crates.io](https://github.com/firecracker-microvm/firecracker/issues/4162).
+
+`dbs-snapshot` provides a version tolerant serialization and deserialization facilities and implements a persistent storage format for saving rust struct states. It can form a basis of many useful functionalities such as VM snapshot for rust-based VMMs, as well as user space program live upgrade for other projects like [Nydus](https://github.com/dragonflyoss/image-service/).
+
+# Disclaimer
+
+Please note that the crate is released from v1.5.0 because that is the initial Firecracker release version the crate was taken from.
+
+We will use sematic versioning. The API backward-compatibilites will always be reserved across patch version updates. The backward-compatibilities across minor version updates will be preserved in a best effort manner. Whenever API backward-compatibility is broken on purpose, we will bump the major version.