fix doc

Author: Loic Descotte

docs/default_values.md

@@ -15,8 +15,13 @@ val noneList: Option[List[String]] = None
 noneList.orEmpty  // List[String]()
 ```
 
-To support other types, you just have to declare a new implicit value : 
+To support other types, you have to declare a new implicit monoid for this type.
+
+Example with the BigDecimal type (already supported) : 
 
  ```scala
-implicit def myCaseClassDefaultValue = new DefaultValue[MyCaseClass] { ... }
-```
+implicit val bigDecimalMonoid: Monoid[BigDecimal] = new Monoid[BigDecimal] {
+  override def empty: BigDecimal = BigDecimal(0)
+  override def compose(l: BigDecimal, r: BigDecimal): BigDecimal = l + r
+}
+```