Add README for programming the STM board

STM32F04_FW/README.md

+# Program Firmware
+Programming the firmware requires the `st-link` util (see
+[https://github.com/texane/stlink](https://github.com/texane/stlink) for
+further information). If installed, flashing the firmware is done with the
+command `make flash`.
+
+Alternatively, ST's graphical interface can be used
+([https://www.st.com/en/development-tools/stsw-link004.html](https://www.st.com/en/development-tools/stsw-link004.html)).