Add release notes link to CHANGELOG
-
### 2.8.0 (2019-09-19)
- Criteria & Repositories API
- Not a bugfix release yet, expect more bugfixes in future 2.8.x versions
+- [2.8.0 release notes](https://github.com/immutables/immutables.github.io/blob/204c4ada80dcb42226b448294f1e90a36524ce52/2.8.0-release.md)
### 2.7.4 (2018-12-26)
- Bugfix and refinements release! Thank you issue reports and PRs!
