Add example for using ESP.rebootIntoUartDownloadMode()

[mhightower83]

For the case when your ESP8266 module or IoT device does not have the nice reset and program circuitry, the proxy method shown in this example can help. At least for upgrades from stable code running with this proxy.

@twischer thank you for sharing your work. 👍
I forgot that I had wanted to do this proxy a long time ago; however, I could not find a start flash download API like yours to call.
And at that time, I was not ready for the deep digging to make it work as you did. This would have been/will be real handy when reflashing the Sonoff SV and Basic.

[twischer]

@mhightower83

This would have been/will be real handy when reflashing the Sonoff SV and Basic.

This was also my main reason for this feature.
Great idea to enter UART download mode when ESP_SYNC was detected. This should be already unique enough. I was thinking about any unique string to sent but using ESP_SYNC itself is much better.
I am quite excited to port this solution even for https://github.com/arendst/Tasmota

[d-a-v]

LGTM!