Fix documentation on how to compile for Arduino

[Zvezdin]

Hey, thank you for the neat library! I had a quite hard time getting it to compile on the ESP8266 and eventually narrowed it down to three issues:

Adding a -fexeptions flag in file Arduino15/packages/esp8266/hardware/esp8266/3.1.2/boards.txt, line generic.menu.exception.disabled.build.exception_flags=-fexceptions (or the board variant if not generic). No need to edit the platform.txt file.

Removing the StringSumHelper constructor in string.h

Applying #15

It'd be great if there's a guide for that :)

[Zvezdin]

Also, I found an issue with istream_Stream::readuntil, it expects every line to be at most 81 chars long, which is according to the standard but Nextcloud's webdav client creates lines up to 90 in my case. So I'd recommend increasing that buffer to 100.

[sourcesimian]

Thanks for the feedback. I'm happy to review and accept pull requests, but I am not actively working this repo at this time.