Add changelog entry

Author: mduesterhoeft

packages/apollo-gateway/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._
+- __FIX__: Continue resolving when an `@external` reference cannot be resolved. [#3914](https://github.com/apollographql/apollo-server/pull/3914)
 
 ## 0.16.5