trackpad-fw: Add README.md

Document the new automatic bootloader mode, and some other details as
well.

reform2-trackpad-fw/README.md

+# MNT Reform 2.0 Touchpad Firmware
+
+## Code Structure
+
+- `Mouse.c`: Sensor gesture processing + USB HID report generation
+- `azoteq.{c,h}`: Azoteq sensor registers in structs for easy access
+- `Descriptors.c`: USB HID descriptors
+
+### Debian/Ubuntu
+
+`apt install gcc-avr avr-libc dfu-programmer`
+
+### Mac
+
+*TODO: is this correct?*
+
+```
+brew tap osx-cross/avr
+brew install avr-gcc
+brew install dfu-programmer
+```
+
+## Building
+
+Build the firmware by running `make`. The firmware can then be found in
+Mouse.hex.
+
+# Analysis
+
+After building, you can load `Mouse.elf` in `gdb`, and inspect the generated code.
+Try `disas main`, for example.
+
+To flash, run:
+`sudo ./flash.sh`
+
+The script sends the 'xJTBL' command to the trackpad to send it into bootloader mode, and then flashes the new firmware.
+The trackpad should reset briefly after flashing, and start running the new firmware.