Add to usage

Author: benjie

README.md

@@ -187,3 +187,20 @@ import { schema as sourceSchema } from "./my-schema";
 
 export const schema = semanticToStrict(sourceSchema);
 ```
+
+## Advanced usage
+
+If you just want to convert a single `GraphQLFieldConfig` you can use the
+`convertFieldConfig` method, passing the field config and `true` to convert
+semantic non-null positions to strict non-nulls, or `false` if you want to
+convert to nullable:
+
+```ts
+const strictFieldConfig = convertFieldConfig(fieldConfig, true);
+const nullableFieldConfig = convertFieldConfig(fieldConfig, false);
+```
+
+> [!NOTE]
+>
+> This method assumes that the fieldConfig has come from parsing an SDL string,
+> and thus has an `astNode` that includes a `@semanticNonNull` directive.