update readme

Author: newpavlov

README.md

@@ -19,13 +19,51 @@ Crates have not yet received any formal cryptographic and security reviews.
 | `cfb-mode` | [![crates.io](https://img.shields.io/crates/v/cfb-mode.svg)](https://crates.io/crates/cfb-mode) | [![Documentation](https://docs.rs/cfb-mode/badge.svg)](https://docs.rs/cfb-mode) |
 | `cfb8` | [![crates.io](https://img.shields.io/crates/v/cfb8.svg)](https://crates.io/crates/cfb8) | [![Documentation](https://docs.rs/cfb8/badge.svg)](https://docs.rs/cfb8) |
 | `ctr` | [![crates.io](https://img.shields.io/crates/v/ctr.svg)](https://crates.io/crates/ctr) | [![Documentation](https://docs.rs/ctr/badge.svg)](https://docs.rs/ctr) |
+| `ofb` | [![crates.io](https://img.shields.io/crates/v/ofb.svg)](https://crates.io/crates/ofb) | [![Documentation](https://docs.rs/ofb/badge.svg)](https://docs.rs/ofb) |
 
 
 ### Minimum Rust version
 All crates in this repository support Rust 1.27 or higher. In future minimum
 supported Rust version can be changed, but it will be done with the minor
 version bump.
 
+## Usage
+
+Crates functionality is expressed in terms of traits defined in the
+[`stream-cipher`][2] crate.
+
+Let's use AES-128-OFB to demonstrate usage of synchronous stream cipher:
+```rust
+extern crate aes;
+extern crate ofb;
+
+use aes::Aes128;
+use ofb::Ofb;
+// import relevant traits
+use ofb::stream_cipher::{NewStreamCipher, SyncStreamCipher};
+
+// OFB mode implementation is generic over block ciphers
+// we will create a type alias for convenience
+type AesOfb = Ofb<Aes128>;
+
+let key = b"very secret key.";
+let iv = b"unique init vect";
+let plaintext = b"The quick brown fox jumps over the lazy dog.";
+
+let mut buffer = plaintext.to_vec();
+// create cipher instance
+let mut cipher = AesOfb::new_var(key, iv)?;
+// apply keystream (encrypt)
+cipher.apply_keystream(&mut buffer);
+// and decrypt it back
+AesOfb::new_var(key, iv)?.apply_keystream(&mut buffer);
+// stream ciphers can be used with streaming messages
+let mut cipher = AesOfb::new_var(key, iv).unwrap();
+for chunk in buffer.chunks_mut(3) {
+    cipher.apply_keystream(chunk);
+}
+```
+
 ## License
 
 All crates licensed under either of
@@ -42,3 +80,4 @@ 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.
 
 [1]: https://en.wikipedia.org/wiki/Stream_cipher
+[2]: https://docs.rs/stream-cipher