Document Atom's `require` and `#include` functionality

[Benjamin-Dobell]

There's presently no documentation for require. Even #include was never documented properly, there was only https://github.com/Berserk-Games/atom-tabletopsimulator-lua/wiki/%23include which doesn't really go into settings, and it requires users to know about Github.

[Benjamin-Dobell]

Should also document <Include src="blah"/> XML includes.

[dore0062]

You can do XML includes? Oh man.... the amount of time that would have saved me. I might work on doing this after some testing.

[omniraptorr]

this should be edited into docs/atom.md