Add a TODO document

doc/todo.md

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+
+# Things to do and random ideas
+
+## Hardware Change Ideas
+
+* Connectors for I2C,SPI,neopixel ...
+  * Could double as a connector for a small daughterboard
+* Fine pitch connector for the debug connector
+* Tiny buttons for rst/boot1 ...
+* On board g-force/gyro chip, or as a small daughterboard
+* Change to a 5V DCDC and a linear 3.3V?  To provice 5V for things like LEDs and servos.
+
+### Software
+
+* Develop a CAN bus capable bootloader for flashing over CAN. Probably needs incompatible changes to the CAN protocol
+* Improve the CAN bus protocol to be more bandwidth efficient. Such as rely on CAN CRC and remove klipper CRC
+* Modify klipper firmware to keep USB usable when using CAN bus
+ * At least allow USB to detect boot loader trying to flash
+ * Automatic switching to/from USB when connected
+
+#### Bugs/flaws
+
+* It's not possible to enter DFU bootloader when in klipper firmware automatically. The SWD pins might be in the wrong state.
+* The CanSerial utility/Linux CAN bus interface is not as stable as you might wish
+
+### Hardware issues
+
+* The TPS54308 DCDC converter generates a lot of heat at idle. Is there a better way to get 3.3V with low ripple?
+
+## Hardware Cost Cutting
+
+* Use an STM32F0
+ * no crystal needed? Even for CAN?
+* Change to some cheap generic DCDC controller.