docs: add reference section index

Signed-off-by: perigoso <perigoso@riseup.net>

docs/reference/index.rst

@@ -0,0 +1,64 @@
+*********
+Reference
+*********
+
+.. figure:: ../assets/stack.png
+   :width: 1600px
+   :alt: stackup
+
+   representation of the TinyUSB stack.
+
+Device Stack
+============
+
+Supports multiple device configurations by dynamically changing usb descriptors. Low power functions such like suspend, resume, and remote wakeup. Following device classes are supported:
+
+-  Audio Class 2.0 (UAC2)
+-  Bluetooth Host Controller Interface (BTH HCI)
+-  Communication Class (CDC)
+-  Device Firmware Update (DFU): DFU mode (WIP) and Runtinme
+-  Human Interface Device (HID): Generic (In & Out), Keyboard, Mouse, Gamepad etc ...
+-  Mass Storage Class (MSC): with multiple LUNs
+-  Musical Instrument Digital Interface (MIDI)
+-  Network with RNDIS, CDC-ECM (work in progress)
+-  USB Test and Measurement Class (USBTMC)
+-  Vendor-specific class support with generic In & Out endpoints. Can be used with MS OS 2.0 compatible descriptor to load winUSB driver without INF file.
+-  `WebUSB <https://github.com/WICG/webusb>`__ with vendor-specific class
+
+If you have special need, `usbd_app_driver_get_cb()` can be used to write your own class driver without modifying the stack. Here is how RPi team add their reset interface [raspberrypi/pico-sdk#197](https://github.com/raspberrypi/pico-sdk/pull/197)
+
+Host Stack
+==========
+
+.. admonition:: Warning
+   :class: warning
+
+   Most active development is on the Device stack. The Host stack is under rework and largely untested.
+
+- Human Interface Device (HID): Keyboard, Mouse, Generic
+- Mass Storage Class (MSC)
+- Hub currently only supports 1 level of hub (due to my laziness)
+
+OS Abstraction layer
+====================
+
+TinyUSB is completely thread-safe by pushing all ISR events into a central queue, then process it later in the non-ISR context task function. It also uses semaphore/mutex to access shared resources such as CDC FIFO. Therefore the stack needs to use some of OS's basic APIs. Following OSes are already supported out of the box.
+
+- **No OS**
+- **FreeRTOS**
+- **Mynewt** Due to the newt package build system, Mynewt examples are better to be on its [own repo](https://github.com/hathach/mynewt-tinyusb-example)
+
+License
+=======
+
+All TinyUSB sources in the `src` folder are licensed under MIT license. However, each file can be individually licensed especially those in `lib` and `hw/mcu` folder. Please make sure you understand all the license term for files you use in your project.
+
+Index
+=====
+
+.. toctree::
+   :maxdepth: 2
+
+   supported
+   getting_started
+   concurrency