Add basic documentation to readme

README.md

+# Programmer UPDI FT230XS
+
+Serial (FTDI) programmer/debugger based on the FT230XS FTDI chip. No programming needed. Works out of the box.
+
+## Features
+
+- Minimal BOM
+- No pre-programming required
+
+## Pinout
+
+![Pinout image](img/programmer-serial-ft230xs_pinout.jpg)
+
+## Board
+
+> New in KiCAD 6.0: interactive HTML BOM plugin (download [bom/ibom.html](bom/ibom.html) and open the file in any browser).
+
+![Board image](img/programmer-serial-ft230xs_board.jpg)
+
+## Schematic
+
+![Board image](img/programmer-serial-ft230xs_schematic.jpg)
+
+## Using as a Debugger
+
+The primary purpose of this board is to use it as serial debugger for the ATtiny 0/1/2 chips. Boards should feature a FTDI connector and have to be programmed to use the serial port for communication.
+
+## Using as a Programmer
+
+You will need a serial (FTDI) to UPDI adapter to do this. **TODO: Add link to adapter board in collection when ready.** It uses a resistor to combine RX and TX pins into one single UPDI line.
+
+You can program AVR targets that support UPDI by simply connecting the UPDI pin and uploading the binary through pyupdi, pymcuprog or directly through the Arduino IDE if using megaTinyCore or DxCore. In the latter case, the following settings must be selected in Tools:
+
+- Port: FTDI serial port
+- Programmer: SerialUPDI - SLOW
+
+> Make sure that the Board setting matches your target chip.