add info about blueprint upgrade to changelog

Author: haschek

CHANGELOG.md

@@ -29,6 +29,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/) and this p
 - **React and its types were updated to v18, so you may hit incompatibilities if you run it with React 16 or 17.**
 - `color` library was upgraded from v4 to v5, so the types changed
     - if you forward properties then they cannot have `Color` as type, use `ColorLike`
+- `@blueprintjs/core` library was updated to v6
+    - you may need to update class names in your tests (the new prefix is `bp6-`)
 - `<MultiSelect />`
     - by default, if no searchPredicate or searchListPredicate is defined, the filtering is done via case-insensitive multi-word filtering.