esp32/README.md: Add note about btree submodule initialization.
esp32/README.md: Add note about btree submodule initialization.

MrSurly/micropython-esp32,MrSurly/micropython-esp32,MrSurly/micropython-esp32,MrSurly/micropython-esp32,micropython/micropython-esp32,micropython/micropython-esp32,micropython/micropython-esp32,micropython/micropython-esp32,micropython/micropython-esp32,MrSurly/micropython-esp32