Updte README.md, add demonstration

README.md

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+# Slideshow
+
+This project is a relatively simple slideshow running on the Adafruit Feather
+328P (an ATmega328p-microcontroller) using a simple button and a 240x240 LCD
+display with the ST7789-driver over
+[SPI](https://en.wikipedia.org/wiki/Serial_Peripheral_Interface).
+
+This project is written on Rust and uses a very minimal amount of libraries with
+the most significant being [`atmega-hal`](https://github.com/Rahix/avr-hal), a
+hardware abstraction layer for ATmega-microcontrollers. Images are stored in the
+[program memory](https://en.wikipedia.org/wiki/Flash_memory) of the device and
+manually read using the `lpm`-instruction using inline-assembly because Rust
+itself does not produce this instruction in any scenario. The images are stored
+in the [QOI-specification](https://qoiformat.org/qoi-specification.pdf) to save
+space, and they are decoded on-the-fly when rendering the images. Communication
+between the display and the microprocessor is done manually using a [minimal
+SPI-utility](https://rahix.github.io/avr-hal/atmega_hal/spi/type.Spi.html)
+within the `atmega-hal` crate.
+
+## Demonstration
+
+![demonstration](./demonstration.mp4)