Document graphql macro attributes

[llGaetanll]

I was able to find out about attributes in the code here, but found them nowhere in the docs (maybe I'm just not looking in the right place?)

let variables_derives = attributes::extract_attr(input, "variables_derives").ok();
let response_derives = attributes::extract_attr(input, "response_derives").ok();
let custom_scalars_module = attributes::extract_attr(input, "custom_scalars_module").ok();
let extern_enums = attributes::extract_attr_list(input, "extern_enums").ok();
let fragments_other_variant: bool = attributes::extract_fragments_other_variant(input);

It would be very helpful for new users to document which attributes are available!

[tomhoule]

I agree the docs could get some more love. I'd be happy to review a PR.