Add a minimal README.md explaining the design rationale.

Author: adamreichold

README.md

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+# serde-mincode
+
+[![crates.io](https://img.shields.io/crates/v/serde-mincode.svg)](https://crates.io/crates/serde-mincode)
+[![docs.rs](https://docs.rs/serde-mincode/badge.svg)](https://docs.rs/serde-mincode)
+[![github.com](https://github.com/adamreichold/serde-mincode/actions/workflows/rust.yml/badge.svg)](https://github.com/adamreichold/serde-mincode/actions/workflows/rust.yml)
+
+A minimal binary encoding of Serde-compatible types similar in spirit to the original [bincode](https://docs.rs/bincode/1.3.3/bincode/) yet with fewer options and without I/O integration.
+
+## License
+
+Licensed under
+
+ * [Apache License, Version 2.0](LICENSE-APACHE) or
+ * [MIT license](LICENSE-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.