UG: Add help window known issue

[hjungwoo01]

Fixes #24.

[brein62]

LGTM! This describes the known issue for the help window clearly and succinctly enough, as well as describes a fix that can be used across all operating systems.

[hjungwoo01]

@damithc would it also be helpful to state that not only will it not work when you type in the help command, but it will also not work when you click on the Help button?

[damithc]

@damithc would it also be helpful to state that not only will it not work when you type in the help command, but it will also not work when you click on the Help button?

Good point, @hjungwoo01 There is the F1 route too. Something like this?

... run the help command (or use the Help menu, or the keyboard shortcut F1) ...

[damithc]

Thanks for the PR @hjungwoo01
I added a commit message.