Post-release: Update CHANGELOG with references to publish commit.

So that users understand all the related versions of updated packages, which
don't necessarily match up with those of Apollo Server itself (i.e. non
`apollo-server` packages don't use the same versions, so make that more
clear.).

CHANGELOG.md

@@ -10,12 +10,16 @@ The version headers in this history reflect the versions of Apollo Server itself
 
 ### v2.6.6
 
+> [See complete versioning details.](https://github.com/apollographql/apollo-server/commit/26db63cbd5adf54b07a5b67c0e0fbff8e61c79aa)
+
 - `apollo-server-core`: Avoid duplicate `cacheControl` directives being added via `isDirectiveDefined`, re-landing the implementation reverted in v2.6.1 which first surfaced in v2.6.0. [PR #2762](https://github.com/apollographql/apollo-server/pull/2762) [Reversion PR #2754](https://github.com/apollographql/apollo-server/pull/2754) [Original PR #2428](https://github.com/apollographql/apollo-server/pull/2428)
 - `apollo-server-testing`: Add TypeScript types for `apollo-server-testing` client. [PR #2871](https://github.com/apollographql/apollo-server/pull/2871)
 - `apollo-server-plugin-response-cache`: Fix undefined property access attempt which occurred when an incomplete operation was received. [PR #2792](https://github.com/apollographql/apollo-server/pull/2792) [Issue #2745](https://github.com/apollographql/apollo-server/issues/2745)
 
 ### v2.6.5
 
+> [See complete versioning details.](https://github.com/apollographql/apollo-server/commit/a2b2a0d8f013826d08433129a69834035e04f1d5)
+
 - `apollo-engine-reporting`: Simplify the technique for capturing `operationName`. [PR #2899](https://github.com/apollographql/apollo-server/pull/2899)
 - `apollo-server-core`: Fix regression in 2.6.0 which caused `engine: false` not to disable Engine when the `ENGINE_API_KEY` environment variable was set. [PR #2850](https://github.com/apollographql/apollo-server/pull/2850)
 - `@apollo/federation`: Introduced a `README.md`. [PR #2883](https://github.com/apollographql/apollo-server/pull/2883)