Add changelog entry for #256

Author: LegNeato

juniper_hyper/Changelog.md

@@ -0,0 +1,8 @@
+# [master]
+
+## Breaking changes
+
+- The tokio threadpool managed by `hyper` is now used for
+  executing GraphQL operations as well. Previously a separate threadpool from `futures_cpupool` was required.
+
+  [#256](https://github.com/graphql-rust/juniper/pull/256)