Update ChangeLog

Author: eysk-io

extensions/ql-vscode/CHANGELOG.md

@@ -3,6 +3,7 @@
 ## [UNRELEASED]
 
 - Display CodeQL CLI version being downloaded during an upgrade. [#862](https://github.com/github/vscode-codeql/pull/862)
+- Display a helpful message and link to documentation when a query produces no results. [#866](https://github.com/github/vscode-codeql/pull/866)
 
 ## 1.4.8 - 05 May 2021