Moved the alias option to optional instead of under annotation in the readme

Author: marcojanssen

README.md

@@ -212,8 +212,10 @@ Configuration
      * **resources_namespace**: A namespaceish path to where the mapping
        files are located. Example: `Path\To\Foo\Resources\mappings`
 
-     Each **annotation** mapping may also specify the following options:
+     Each mapping definition can have the following optional options:
      * **alias** (Default: null): Set the alias for the entity namespace.
+
+     Each **annotation** mapping may also specify the following options:
      * **use_simple_annotation_reader** (Default: true):
        If `true`, only simple notations like `@Entity` will work.
        If `false`, more advanced notations and aliasing via `use` will