Add changelog entry for apollographql/apollo-server#4209

Apollo-Orig-Commit-AS: apollographql/apollo-server@7354696

federation-js/CHANGELOG.md

@@ -4,7 +4,7 @@
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
-- _Nothing yet! Stay tuned._
+- Remove federation primitives from SDL during composition. This allows for services to report their *full* SDL from the `{ _service { sdl } }` query as opposed to the previously limited SDL without federation definitions. [#4209](https://github.com/apollographql/apollo-server/pull/4209)
 
 ## 0.16.4