add FAQ entry for huge-interface generated files

Author: WillAbides

docs/faq.md

@@ -20,6 +20,10 @@ Tell genqlient how to handle your custom scalars with the [`bindings` option](sc
 
 Yes, but you'll need to use a separate tool ([example](schema.md#fetching-your-schema)).
 
+### My generated file is huge because my interface has many implementations
+
+Set [`omit_unreferenced_implementations`](genqlient.yaml) in `genqlient.yaml`. See [operations.md](operations.md#graphql-interfaces).
+
 ## Why?
 
 ### Why use genqlient?