Note to self: To force WeMos D1 (cheap Arduino ESP8266 board) into BOOT mode, pull down GPIO0. To pull it down, put a resistor between GPIO0 and GND. I use 100ohm, but anything lower than the built-in pullup resistor should work. GPIO0 is pin D8 on the old "WeMos D1 R1", and pin D3 on the newer "WeMos D1 R2".