readme, doc comments, clippies

README.md

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+# usbd-uac2
+
+USB Audio Class 2.0
+
+This crate provides a USB Audio Class 2.0 implementation for
+[usb-device](https://crates.io/crates/usb-device). It implements all
+required elements of the specification, however many controls are not
+implemented (e.g. mixers, effects).
+
+Device behaviour is driven by implementing the `ClockSource` and
+`AudioHandler` traits to configure the audio pipeline and source/sink data
+to/from USB.
+
+Example (creates a UAC2 device with in and out streams):
+
+```rust
+let mut audio = YourTraitImpl {...};
+
+let config = UsbAudioClassConfig::new(usb_speed, FunctionCode::IoBox, &mut audio)
+    // base_id is USB entity ID, id 1 is always taken by the clock source, and each stream builds 2 entities
+    .with_output_config(TerminalConfig::builder().base_id(2).build())
+    .with_input_config(TerminalConfig::builder().base_id(4).build());
+
+let mut uac2 = config.build(&usb_bus).unwrap();
+
+let mut usb_dev = usbd_uac2::builder(&usb_bus, UsbVidPid(0x1209, 0x0001))
+    .strings(&[StringDescriptors::default()
+        .manufacturer("usbd_uac2")
+        .product("example")])
+    .unwrap()
+    .max_packet_size_0(64) // Required to be 64 on HS
+    .unwrap()
+    .build();
+```
+
+No work needs to be done in the poll loop, the class implementation will
+call your trait callbacks as required, just call the usb poll as usual:
+
+```rust
+loop {
+    usb_dev.poll(&mut [&mut uac2]);
+}
+```
+
+See the trait documentation or examples for additional details.