Add section on operators to the reference docs

[gabejohnson]

It's covered in the tutorial but should probably be in the reference docs as well.

[Jamesernator]

Not specified in the tutorial and should be in the docs is whether or not you can export/import them.

I tried export operator + ... but nothing happened when I tried import { + } from .... It didn't raise an error so I'm not sure what the intended behaviour is given that I can also include a regular export syntax ... within the same file and it got applied fine.

[vendethiel]

@Jamesernator #711 was merged but not released yet (i think).

[disnet]

@vendethiel released: https://github.com/sweet-js/sweet-core/releases/tag/v3.0.8