#10 add documentation

Author: hduelme

api/src/main/java/org/mapstruct/tools/gem/GemDefinition.java

@@ -26,5 +26,13 @@
      */
     Class<?> value();
 
+    /**
+     * Specifies the name of the implementation class. The {@code <CLASS_NAME>} will be replaced by the
+     * interface/abstract class name.
+     * <p>
+     * Defaults to postfixing the name with {@code Gem}: {@code <CLASS_NAME>Impl}
+     *
+     * @return The implementation name.
+     */
     String implementationName() default "<CLASS_NAME>";
 }