Add readme for async crate

Author: lulf

embedded-storage-async/README.md

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+# embedded-storage-async
+
+> An Embedded Storage Abstraction Layer
+
+This create defines a set of async traits variants of embedded-storage.
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+
+[API reference]: https://docs.rs/embedded-storage-async