Add a changelog entry

Author: weiznich

changelog/master.md

@@ -1,3 +1,16 @@
 # [master] yyyy-mm-dd
 
 ## Changes
+
+- Juniper is now generic about the exact representation of scalar values. This
+  allows downstream crates to add support for own scalar value representations.
+
+   There are two use cases for this feature:
+   * You want to support new scalar types not representable by the provided default
+   scalar value representation like for example `i64`
+   * You want to support a type from a third party crate that is not supported by juniper
+
+  **Note:** This may need some changes in down stream code, especially if working with
+  generic code. To retain the current behaviour use `DefaultScalarValue` as scalar value type
+
+  [#251](https://github.com/graphql-rust/juniper/pull/251)