Add httpupdate lib import in ota documentation

[amjed-ali-k]

Docs doesn't include any reference to include <ESP8266httpUpdate.h> library in Advanced OTA Updater section. It took me some time to find out while reading the docs

[DarioGHub]

@amjed-ali-k I may not understand your motivation, but a lesson I learned was that "docs" include the examples. Given that the examples use the line #include <ESP8266httpUpdate.h>, how is it helpful to add that line twice more to readme.rst? Perhaps you would like to clarify the need.