Add argument validation to GraphQL Styleguide

doc/development/api_graphql_styleguide.md

@@ -636,6 +636,37 @@ found, we should raise a
 `Gitlab::Graphql::Errors::ResourceNotAvailable` error. Which will be
 correctly rendered to the clients.
 
+## Validating arguments
+
+For validations of single arguments, use the
+[`prepare` option](https://github.com/rmosolgo/graphql-ruby/blob/master/guides/fields/arguments.md)
+as normal.
+
+Sometimes a mutation or resolver may accept a number of optional
+arguments, but still want to validate that at least one of the optional
+arguments were given. In this situation, consider using the `#ready?`
+method within your mutation or resolver to provide the validation. The
+`#ready?` method will be called before any work is done within the
+`#resolve` method.
+
+Example:
+
+```ruby
+def ready?(**args)
+  if args.values_at(:body, :position).compact.blank?
+    raise Gitlab::Graphql::Errors::ArgumentError,
+          'body or position arguments are required'
+  end
+
+  # Always remember to call `#super`
+  super(args)
+end
+```
+
+In the future this may be able to be done using `InputUnions` if
+[this RFC](https://github.com/graphql/graphql-spec/blob/master/rfcs/InputUnion.md)
+is merged.
+
 ## GitLab's custom scalars
 
 ### `Types::TimeType`