doc: Add documentation to generate the API client

It will be helpful to have the documentation for generating the API
client in this repository.

This patch adds the documentation as MarkDown file

GENERATE.md

@@ -0,0 +1,23 @@
+# Generating PHP using OpenAPI/Swagger specifications
+
+The BAG API is using the OpenAPI/Swagger 3.0 specification, which means you can have PHP classes generated
+automatically.
+
+The URL for the BAG Swagger UI is https://lvbag.github.io/BAG-API/Technische%20specificatie/
+
+## Prerequisites
+
+* Make sure the JAVA JDK is at version 17 at least
+* Download the latest stable version of the code generator at https://repo1.maven.org/maven2/io/swagger/codegen/v3/swagger-codegen-cli/
+
+## Generate
+
+Go one level before the root of this repository and enter the following in CLI:
+`java -jar swagger-codegen-cli.jar generate -i https://api.bag.kadaster.nl/lvbag/individuelebevragingen/v2/openapi.json -l php -o kadaster-bag-api-client`
+
+## Namespaces
+
+The generator will use the namespace `Swagger\Client` for the generated PHP classes. This is not desired, especially
+when combining multiple Swagger API's. 
+
+Therefor we renamed the namespace to `Kadaster\BapApiClient`