Update README.md

Author: kobylynskyi

README.md

@@ -13,23 +13,39 @@ Please refer to:
 
 ## Supported Options
 
-| Key                       | Data Type          | Default value                         | Description |
-| ------------------------- | ------------------ | ------------------------------------- | ----------- |
-| graphqlSchemaPaths        | List(String)       | None                                  | GraphQL schema locations. You can supply multiple paths to GraphQL schemas. |
-| packageName               | String             | Empty                                 | Java package for generated classes. |
-| outputDir                 | String             | None                                  | The output target directory into which code will be generated. |
-| apiPackage                | String             | Empty                                 | Java package for generated api classes (Query, Mutation, Subscription). |
-| modelPackage              | String             | Empty                                 | Java package for generated model classes (type, input, interface, enum, union). |
-| generateApis              | Boolean            | True                                  | Specifies whether api classes should be generated as well as model classes. |
-| customTypesMapping        | Map(String,String) | Empty                                 | Can be used to supply custom mappings for scalars. <br/> Supports:<br/> * Map of (GraphqlObjectName.fieldName) to (JavaType) <br/> * Map of (GraphqlType) to (JavaType) |
-| customAnnotationsMapping  | Map(String,String) | Empty                                 | Can be used to supply custom annotations (serializers) for scalars. <br/> Supports:<br/> * Map of (GraphqlObjectName.fieldName) to (JavaType) <br/> * Map of (GraphqlType) to (JavaType) |
+| Key                       | Data Type          | Default value                             | Description |
+| ------------------------- | ------------------ | ----------------------------------------- | ----------- |
+| graphqlSchemaPaths        | List(String)       | None                                      | GraphQL schema locations. You can supply multiple paths to GraphQL schemas. |
+| packageName               | String             | Empty                                     | Java package for generated classes. |
+| outputDir                 | String             | None                                      | The output target directory into which code will be generated. |
+| apiPackage                | String             | Empty                                     | Java package for generated api classes (Query, Mutation, Subscription). |
+| modelPackage              | String             | Empty                                     | Java package for generated model classes (type, input, interface, enum, union). |
+| generateApis              | Boolean            | True                                      | Specifies whether api classes should be generated as well as model classes. |
+| customTypesMapping        | Map(String,String) | Empty                                     | Can be used to supply custom mappings for scalars. <br/> Supports:<br/> * Map of (GraphqlObjectName.fieldName) to (JavaType) <br/> * Map of (GraphqlType) to (JavaType) |
+| customAnnotationsMapping  | Map(String,String) | Empty                                     | Can be used to supply custom annotations (serializers) for scalars. <br/> Supports:<br/> * Map of (GraphqlObjectName.fieldName) to (JavaType) <br/> * Map of (GraphqlType) to (JavaType) |
 | modelValidationAnnotation | String             | @javax.validation.<br>constraints.NotNull | Annotation for mandatory (NonNull) fields. Can be null/empty. |
-| modelNamePrefix           | String             | Empty                                 | Sets the prefix for GraphQL model classes (type, input, interface, enum, union). |
-| modelNameSuffix           | String             | Empty                                 | Sets the suffix for GraphQL model classes (type, input, interface, enum, union). |
-| subscriptionReturnType    | String             | Empty                                 | Return type for subscription methods. For example: `org.reactivestreams.Publisher`, `io.reactivex.Observable`, etc. |
-| generateEqualsAndHashCode | Boolean            | False                                 | Specifies whether generated model classes should have equals and hashCode methods defined. |
-| generateToString          | Boolean            | False                                 | Specifies whether generated model classes should have toString method defined. |
-
-### Inspired by
+| modelNamePrefix           | String             | Empty                                     | Sets the prefix for GraphQL model classes (type, input, interface, enum, union). |
+| modelNameSuffix           | String             | Empty                                     | Sets the suffix for GraphQL model classes (type, input, interface, enum, union). |
+| subscriptionReturnType    | String             | Empty                                     | Return type for subscription methods. For example: `org.reactivestreams.Publisher`, `io.reactivex.Observable`, etc. |
+| generateEqualsAndHashCode | Boolean            | False                                     | Specifies whether generated model classes should have equals and hashCode methods defined. |
+| generateToString          | Boolean            | False                                     | Specifies whether generated model classes should have toString method defined. |
+| jsonConfigurationFile     | String             | Empty                                     | Path to an external mapping configuration. |
+
+### External mapping configuration
+
+Provide a path to external file via property `jsonConfigurationFile`
+Sample content of the file:
+
+```json
+{
+  "generateApis": true,
+  "packageName": "com.kobylynskyi.graphql.testconfigjson",
+  "customTypesMapping": {
+    "Price.amount": "java.math.BigDecimal"
+  }
+}
+```
+
+## Inspired by
 [swagger-codegen](https://github.com/swagger-api/swagger-codegen)