Added the 'Optional Dependencies' section, outlining some of the common
gotchas that have appeared on GitHub or in the YNAB forums. So far, I've
touched on YNAB4 windows installer (now optional), Mono/Gecko, multilib
for Arch, and winetricks. I'm sure more will come that we'll be able to
add here.

Signed-off-by: Elliot V <elliotfriend@gmail.com>