Prepare changelog for 3.40 release (#747)

Author: eseliger

CHANGELOG.md

@@ -12,16 +12,25 @@ All notable changes to `src-cli` are documented in this file.
 ## Unreleased
 
 ### Added
-- New command `src debug`. [#731](https://github.com/sourcegraph/src-cli/pull/731)
-- `src lsif upload` now supports the `-gitlab-token` flag. [#721](https://github.com/sourcegraph/src-cli/pull/721)
-- Batch Changes can be applied to Bitbucket Cloud when `src` is used with Sourcegraph 3.40 or later. [#725](https://github.com/sourcegraph/src-cli/pull/725)
 
 ### Changed
 
 ### Fixed
 
 ### Removed
 
+## 3.40.0
+
+### Added
+
+- New command `src debug`. [#731](https://github.com/sourcegraph/src-cli/pull/731)
+- `src lsif upload` now supports the `-gitlab-token` flag. [#721](https://github.com/sourcegraph/src-cli/pull/721)
+- Batch Changes can be applied to Bitbucket Cloud when `src` is used with Sourcegraph 3.40 or later. [#725](https://github.com/sourcegraph/src-cli/pull/725)
+
+### Changed
+
+- src CLI is now built using go 1.18.
+
 ## 3.39.0
 
 ### Added