Added readme with the board modification instructions

README.md

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+# Nucleo-F303RE
+DigitalAudio485 prototype with STM32 Nucleo-F303RE board
+
+## Board modification
+Initial state: STM32 Nucleo-F303RE board with factory settings
+
+### HSE oscillator from X3 crystal
+P.25 OSC clock supply
+
+* [ ] SB54 and SB55 OFF
+* [ ] R35 and R37 soldered
+* [ ] X3 crystal (8 MHz) soldered
+* [ ] C33 and C34 soldered with 22 pF capacitors
+* [ ] SB16 and SB50 OFF (MCO)
+
+### Disable OSC 32kHz
+P.26 LSE not used
+
+* [ ] SB48 and SB49 ON
+* [ ] R34 and R36 removed
+
+### USB-Serial to UART5
+P.26 USART communication
+
+* [ ] SB62 and SB63 must be ON
+* [ ] SB13 and SB14 must be OFF
+