Add CHANGELOG.md for #2714.

Ref: #2714

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### v2.6.0
 
 - `apollo-server-core`: Introduce new `didEncounterErrors` life-cycle hook which has access to unformatted `errors` property on the `requestContext`, which is the first positional paramater that this new request life-cycle receives.  [PR #2719](https://github.com/apollographql/apollo-server/pull/2719)
+- `apollo-server-core`: Allow request pipeline life-cycle hooks (i.e. plugins) to modify the response's `http.status` code (an integer) in the event of an error.  When combined with the new `didEncounterErrors` life-cycle hook (see above), this will allow modifying the HTTP status code in the event of an error.  [PR #2714](https://github.com/apollographql/apollo-server/pull/2714)
 - `apollo-server-lambda`: Set `callbackWaitsForEmptyEventLoop` to `false` for `OPTIONS` requests to return as soon as the `callback` is triggered instead of waiting for the event loop to empty. [PR #2638](https://github.com/apollographql/apollo-server/pull/2638)
 - `apollo-server`: Support `onHealthCheck` in the `ApolloServer` constructor in the same way as `cors` is supported.  This contrasts with the `-express`, `-hapi`, etc. variations which accept this parameter via their `applyMiddleware` methods and will remain as-is.  [PR #2672](https://github.com/apollographql/apollo-server/pull/2672)
 - core: Expose SHA-256 hex hash digest of the Engine API key to plugins, when available, as `engine.apiKeyHash`. [PR# 2685](https://github.com/apollographql/apollo-server/pull/2685)