Update CHANGELOG.md

Author: Dan Stenmark

CHANGELOG.md

@@ -9,7 +9,7 @@
 - `apollo-codegen-scala`
   - <First `apollo-codegen-scala` related entry goes here>
 - `apollo-codegen-swift`
-  - <First `apollo-codegen-swift` related entry goes here>
+  - Add additional type annotations to improve compile times.  [1638](https://github.com/apollographql/apollo-tooling/pull/1638)
 - `apollo-codegen-typescript`
   - <First `apollo-codegen-typescript` related entry goes here>
 - `apollo-env`