📝Add moment-add and moment-subtract helper usage to docs

[TristanToye]

I have been using this package for a long time and didn't know these helpers existed/got added! Took a few minutes to add come docs when I saw the open issue.

Let me know if anything else should be mentioned.

This closes https://github.com/stefanpenner/ember-moment/issues/300

[jasonmit]

Much appreciated 🙏