README: add project description

Add project description README file.

README.md

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+# CANbardo
+
+CANbardo is an open hardware Universal Serial Bus (USB) to Controller
+Area Network (CAN) adapter board. It is designed to be compatible with
+the open source
+[CANnectivity](https://github.com/CANnectivity/cannectivity) firmware.
+
+CANbardo is licensed under the [CERN-OHL-P-2.0 license](LICENSE).
+
+![CANbardo 3D-view](Documents/3D_view.png)
+
+## Hardware Features
+
+The CANbardo board provides the following features:
+
+- Atmel SAME70N20B microcontroller
+- 12 MHz 10 ppm crystal
+- CAN classic support
+- CAN FD (flexible data rate) support
+- Fast-speed and Hi-speed USB 2.0 support (USB-C connector)
+- Two independent CAN channels (DB-9M connectors)
+- 2 x GPIO-controlled MCP2558FD CAN transceivers
+- 2 x GPIO-controlled 120 ohm CAN bus termination resistors
+- GPIO-controlled LEDs:
+  - 2 x CAN channel state LEDs (green)
+  - 2 x CAN channel activity LEDs (yellow)
+  - 2 x CAN channel error LEDs (red)
+  - 1 x Device Firmware Upgrade (DFU) LED (yellow)
+- Power LED (green)
+- Device Firmware Upgrade (DFU) push button
+- Serial Wire Debug (SWD) and UART (3V3) available on header
+
+## Design Documentation
+
+The following design documentation is available:
+
+- [Schematic](Documents/CANbardo_Schematic.pdf)
+- [Layout](Documents/CANbardo-Layout.pdf)
+- [3D model](Documents/CANbardo.step)
+- [KiCad files](Kicad_8_files)
+- [Fabrication files](Fabrication_files)