Add the readme.

README.md

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+# gravatar-rs
+
+> Gravatar image URL library
+
+## API
+
+For full documentation see [docs.rs].
+
+[docs.rs]: https://docs.rs/gravatar-rs
+
+```rust
+use gravatar_rs::Generator;
+
+let generator = Generator::default();
+
+let gravatar_url = generator.generate("helllo@holllo.cc");
+
+assert_eq!(
+  gravatar_url,
+  "https://www.gravatar.com/avatar/ebff9105dce4954b1bdb57fdab079ff3"
+);
+```
+
+## License
+
+This project is licensed under either of [Apache License, Version 2.0](https://github.com/Holllo/gravatar-rs/blob/main/LICENSE-Apache) or [MIT license](https://github.com/Holllo/gravatar-rs/blob/main/LICENSE-MIT) at your option.
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in either crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.