Update gradle plugin's Readme.md (#1356)

Add a note about use of `systemProperties` since some options are confusing and hard to guess how to use correctly. The only source I was able to find how to configure it in the way I need was a comment to issue https://github.com/OpenAPITools/openapi-generator/issues/551#issuecomment-411686091

modules/openapi-generator-gradle-plugin/README.adoc

@@ -278,6 +278,32 @@ in others being disabled. That is, OpenAPI Generator considers any one of these
 For more control over generation of individual files, configure an ignore file and refer to it via `ignoreFileOverride`.
 ====
 
+[NOTE]
+====
+When configuring `systemProperties` in order to perform selective generation you can disable generation of some parts by providing `"false"` value:
+[source,groovy]
+----
+openApiGenerate {
+    // other settings omitted
+    systemProperties = [
+        modelDocs: "false",
+        apis: "false"
+    ]
+}
+----
+When enabling generation of only specific parts you either have to provide CSV list of what you particularly are generating or provide an empty string `""` to generate everything. If you provide `"true"` it will be treated as a specific name of model or api you want to generate.
+[source,groovy]
+----
+openApiGenerate {
+    // other settings omitted
+    systemProperties = [
+        apis: "",
+        models: "User,Pet"
+    ]
+}
+----
+====
+
 === openApiValidate
 
 .Options