Add a subscription example to the graphql example

[liberatomota-itcenter]

Hello,

I'm not 100% sure that this request makes sense but here it goes:
Having the project being "inspired by Meteor's accounts suite of packages", it would be great if a subscription example was added to the documentation, or even better, a subscription was already added to the types and resolvers.

I don't have the expertise to make a PR with that. I am still trying to make a subscription work with the example code...

Anyway, I think it would be a great advertising for this account system, especially for the ones who add the fortune to work with the meteor accounts packages.

Keep the good work!
Liberato