Graphql-ruby: Missing example on how to **use** interface

[rafaelsales]

This is found in the Objects section, but it just makes harder having to reach another section to see how to use interfaces. I'd add an example on how to use the interface in both links below:

https://github.com/rmosolgo/graphql-ruby/blob/master/guides/type_definitions/interfaces.md
http://graphql-ruby.org/type_definitions/interfaces.html

If you agree with that, I can add submit a PR. Thanks!

[rmosolgo]

Yes, that sounds like a nice improvement, a PR would be great!

[rmosolgo]

Clarified in 90474a965, thanks for reporting the issue!