Add license and readme files

LICENSE.md

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+Copyright (C) 2019 Paul van Tilburg <paul@luon.net>
+===================================================
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
+IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

README.md

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+stm32f4disc-demo
+================
+
+This is some STM32F4DISCOVERY demo application.
+It a serial command-interface for controlling (via GPIO) what the LED
+ring does: cycle clock-wise, counter clock-wise, or follow the accelerometer.
+The accelerometer is accessed via SPI.
+
+Serial interface
+----------------
+
+The serial interface configured on USART 2 can be accessed using, for example,
+an USB-to-serial cable connected to a ground pin, and RX to PA2 and TX to
+PA3.
+
+The interface will output the following lines:
+
+* `init` after initialization has finished
+* `button` when the user button has been pressed
+* `level` when the board is being held in a perfect level position (when in
+   acceleration mode)
+   
+The following lines can be given as commands:
+
+* `on` to turn all the leds on (and disable accelerometer/cycle mode)
+* `off` to turn all the leds off (and disable accelerometer/cycle mode)
+* `accel` to switch to accelerometer mode
+* `cycle` to switch to cycle mode
+* `stop` to freeze the LEDs in the current position
+
+License
+-------
+
+[0-clause BSD license](LICENSE.md).