Mention BlockString class move in CHANGELOG.md

Author: spawnia

CHANGELOG.md

@@ -19,6 +19,7 @@ You can find and compare releases at the [GitHub release page](https://github.co
 - Move debug entries in errors under `extensions` key
 - Use native PHP types for `Schema` and `SchemaConfig`
 - Always throw `RequestError` with useful message when clients provide an invalid JSON body
+- Move class `BlockString` from namespace `GraphQL\Utils` to `GraphQL\Language`
 
 ### Added