Add changelog entry

Author: mduesterhoeft

packages/apollo-gateway/CHANGELOG.md

@@ -3,7 +3,7 @@
 ## vNEXT
 
 > 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.
-
+- __FIX__: Continue resolving when an `@external` reference cannot be resolved. [#3914](https://github.com/apollographql/apollo-server/pull/3914)
 - __FIX__: Collapse nested required fields into a single body in the query plan. Before, some nested fields' selection sets were getting split, causing some of their subfields to be dropped when executing the query. This fix collapses the split selection sets into one. [#4064](https://github.com/apollographql/apollo-server/pull/4064)