Add new option to maven plugin's readme (#3025)

Add new option to maven plugin's readme

Author: wing328

modules/openapi-generator-maven-plugin/README.md

@@ -83,6 +83,7 @@ mvn clean compile
 - `additionalProperties` - sets additional properties that can be referenced by the mustache templates in the format of name=value,name=value. You can also have multiple occurrences of this option
 - `reservedWordsMappings` - specifies how a reserved name should be escaped to. Otherwise, the default `_<name>` is used. For example `id=identifier`. You can also have multiple occurrences of this option
 - `skipIfSpecIsUnchanged` - Skip the execution if the source file is older than the output folder (`false` by default. Can also be set globally through the `codegen.skipIfSpecIsUnchanged` property)
+- `engine` - The name of templating engine to use, "mustache" (default) or "handlebars" (beta)
 
 
 ### Custom Generator