qrcode: Add a readme

matrix_qrcode/README.md

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+[![Build Status](https://img.shields.io/travis/matrix-org/matrix-rust-sdk.svg?style=flat-square)](https://travis-ci.org/matrix-org/matrix-rust-sdk)
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+
+# matrix-qrcode
+
+**matrix-qrcode** is a crate to easily generate and parse QR codes for
+interactive verification using [QR codes] in Matrix.
+
+[Matrix]: https://matrix.org/
+[Rust]: https://www.rust-lang.org/
+[QR codes]: https://spec.matrix.org/unstable/client-server-api/#qr-codes
+
+## Usage
+
+This is probably not the crate you are looking for, it's used internally in the
+matrix-rust-sdk.
+
+If you still want to play with QR codes, here are a couple of helpful examples.
+
+
+### Decode an image
+
+```rust
+use image;
+use matrix_qrcode::{QrVerificationData, DecodingError};
+
+fn main() -> Result<(), DecodingError> {
+    let image = image::open("/path/to/my/image.png").unwrap();
+    let result = QrVerificationData::from_image(image)?;
+
+    Ok(())
+}
+```
+
+### Encode into a QR code
+
+```rust
+use matrix_qrcode::{QrVerificationData, DecodingError};
+use image::Luma;
+
+fn main() -> Result<(), DecodingError> {
+    let data = b"MATRIX\
+        \x02\x02\x00\x07\
+        FLOW_ID\
+        AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\
+        BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB\
+        SHARED_SECRET";
+
+    let data = QrVerificationData::from_bytes(data)?;
+    let encoded = data.to_qr_code().unwrap();
+    let image = encoded.render::<Luma<u8>>().build();
+
+    Ok(())
+}
+```
+
+
+## License
+
+[Apache-2.0](https://www.apache.org/licenses/LICENSE-2.0)