FINERACT-1101 Swagger Codegen build using Gradle

docs/developers/swagger/client.md

@@ -7,20 +7,40 @@ Apache Fineract supports client code generation using [Swagger Codegen](https://
 
 In root directory of the project:
 
-- Run `./gradlew build`
+- Run `./gradlew resolve`
 - Run `./gradlew generateSwaggerCode`
 
 The client code will be generated under `build/swagger-code-fineract`
 
-## Build API Client:
+## Build API Client
 
 - Run `cd build/swagger-code-fineract`
 - Run `./gradlew build`
 
-## Customize Code Generator:
+Alternatively, if you have Maven installed on your system, you can also do:
+
+- Run `cd build/swagger-code-fineract`
+- Run `mvn clean package`
+
+## Customize Generated Code
 
 Swagger Codegen provides several options to customize the generated code. [Here](https://openapi-generator.tech/docs/generators/java/) are the options available for customization.
 
-- Open `fineract-provider/config/swagger/config.json`
+- Open the [config.json.template](https://github.com/apache/fineract/blob/develop/fineract-provider/config/swagger/config.json.template) file
 - Customize options
-- Build the project again as mentioned in **Generate API Client Code** section
+- Build the project again as mentioned in **Generate API Client Code** section
+  
+## Customize using Mustache Templates
+
+Swagger Codegen uses Mustache Templates for generating the client library. For additional customizations you can add/edit custom templates inside the `fineract-provider/config/swagger/templates` folder. 
+
+Make sure you are following the supported templates. Otherwise, the generated code will not build correctly.
+
+- [Java Mustache Templates](https://github.com/swagger-api/swagger-codegen/tree/master/modules/swagger-codegen/src/main/resources/Java)
+- [Retrofit2 Mustache Templates](https://github.com/swagger-api/swagger-codegen/tree/master/modules/swagger-codegen/src/main/resources/Java/libraries/retrofit2)
+
+If you need to add templates for a specific library, you need to follow the same directory structure as present in the links above.
+
+## Validate OpenAPI Spec File
+
+The `resolve` task in [build.gradle](https://github.com/apache/fineract/blob/217e34ba9e4cf04ad7b82fc3a0f8f8e5f8de8042/fineract-provider/build.gradle#L212) file will generate the OpenAPI Spec File for the project. To make sure Swagger Codegen generates a correct library, it is important for the OpenAPI Spec file to be valid. Use [Swagger OpenAPI Validator](https://validator.swagger.io/) to validate the spec file.