Add 1.5.6 header

Author: shati-patel

extensions/ql-vscode/CHANGELOG.md

@@ -1,6 +1,6 @@
 # CodeQL for Visual Studio Code: Changelog
 
-## [UNRELEASED]
+## 1.5.6 - 07 October 2021
 
 - Add progress messages to LGTM download option. This makes the two-step process (selecting a project, then selecting a language) more clear. [#960](https://github.com/github/vscode-codeql/pull/960)
 - Remove line about selecting a language from the dropdown when downloading database from LGTM. This makes the download progress visible when the popup is not expanded. [#957](https://github.com/github/vscode-codeql/pull/957)