Add readme

README.md

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+Setup
+===
+
+1. Need the "newlib" arm library (else there is no stdint.h &co.)
+
+```sh
+pacman -S arm-none-eabi-newlib
+```
+
+2. Get Libopencm3
+
+```sh
+git submodule init
+git submodule update
+```
+
+3. Generate headers in Libopencm3
+
+```sh
+cd libopencm3
+make
+cd ..
+```
+
+Possibly there are also some udev rules & co, I think they can be acquired from
+the `stlink` package.
+
+Usage
+===
+
+```
+make
+```
+
+It does everything. At least for STM32VLDISCOVERY board at my computer.