Document graphql_name configuration for custom directives.

This wasn't documented and I had to dig into the source code to find it.
I tried to follow the style of the rest of the documentation, but feel free to edit however best reflects what you would want.

Author: palasha

guides/type_definitions/directives.md

@@ -39,6 +39,7 @@ Custom directives extend {{ "GraphQL::Schema::Directive" | api_doc }}:
 ```ruby
 # app/graphql/directives/my_directive.rb
 class Directives::MyDirective < GraphQL::Schema::Directive
+  graphql_name "directive_name" # string, not symbol!
   description "A nice runtime customization"
 end
 ```
@@ -54,6 +55,16 @@ class MySchema < GraphQL::Schema
 end
 ```
 
+And you can reference them in the query with `@directive_name(...)`:
+
+```ruby
+query {
+  field @directive_name {
+    id
+  }
+}
+```
+
 {{ "GraphQL::Schema::Directive::Feature" | api_doc }} and {{ "GraphQL::Schema::Directive::Transform" | api_doc }} are included in the library as examples.
 
 ### Arguments